Method for Depositing Noble Metal to Carbon Steel Member of Nuclear Power Plant and Method for Suppressing Radionuclide Deposition on Carbon Steel Member of Nuclear Power Plant

ABSTRACT

A film forming apparatus is connected to a carbon steel purification system piping of a BWR plant (S 1 ). Formic acid (surface purification agent) is injected into a circulation piping of the film forming apparatus (S 4 ). A surface purification agent aqueous solution containing 30000 ppm of formic acid is contacted with the inner surface of the purification system piping, and a large amount of Fe 2+  is dissolved from the purification system piping, and a large amount of electrons are generated by this dissolution. Thereafter, a formic acid Ni aqueous solution is injected into the surface purification agent aqueous solution to produce a film forming aqueous solution (S 5 ). The film forming aqueous solution storing the electrons is contacted with the inner surface of the purification system piping, and Ni ions incorporated into the inner surface are reduced by the electrons, and a Ni metal film is formed on the inner surface. Platinum ions and a reducing agent are injected into the circulation piping (S 9 , S 10 ), and an aqueous solution containing the platinum ions and the reducing agent is supplied to the purification system piping to deposit platinum on the surface of the Ni metal film.

CLAIM OF PRIORITY

The present application claims priority from Japanese Patent application serial No. 2019-019704, filed on Feb. 6, 2019, the content of which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a method for depositing a noble metal on a carbon steel member of a nuclear power plant and a method for suppressing radionuclide deposition on a carbon steel member of a nuclear power plant, and more particularly, to a method for depositing a noble metal on a carbon steel member of a nuclear power plant and a method for suppressing radionuclide deposition on a carbon steel member of a nuclear power plant, suitable for application in a boiling water type nuclear power plant.

2. Description of Related Art

The nuclear power plant such as, for example, a boiling water type nuclear power plant (hereinafter referred to as a BWR plant) and a pressurized water nuclear power plant (hereinafter referred to as a PWR plant) are known. For example, in a BWR plant, steam generated in a reactor pressure vessel (hereinafter referred to as RPV) is guided to a turbine to rotate the turbine. Steam discharged from the turbine is condensed into water by the condenser. This water is supplied as feedwater to the RPV through feedwater piping. In order to suppress generation of the radioactive corrosion product in the RPV, the metal impurities contained in the feedwater are removed with a filtration desalination device provided in the feedwater piping.

In the BWR plant and the PWR plant, in order to suppress corrosion, main component such as RPV is formed of stainless steel, nickel-base alloy, and the like at the water-contacting area that contacts the reactor water (cooling water present in the RPV). From the viewpoint of reducing the cost required for manufacturing the plant or avoiding stress corrosion cracking of stainless steel caused by high temperature water, components such as the reactor purification system, residual heat removal system, and feedwater system connected to the RPV are mainly formed of carbon steel members.

Furthermore, part of the reactor water is purified by the filtration desalination device of the reactor purification system to aggressively remove even a minor presence of metal impurities in the reactor water.

However, despite the corrosion prevention measures described above, there are traces of metal impurities inevitably present in the reactor water, and some of the metal impurities are deposited as metal oxides on the outer surface of the fuel rod contained in the fuel assembly. The metallic elements contained in the metal impurities deposited on the outer surface of the fuel rod cause a nuclear reaction by irradiation of neutrons emitted from the nuclear fuel material in the fuel rod, and produce radionuclide such as cobalt 60, cobalt 58, chromium 51, manganese 54, and the like. Part of the radionuclides deposited in the form of oxides on the outer surface of the fuel rod is dissolved as ions into the reactor water depending on the solubility of the oxides that contain radionuclides, and is released again into the reactor water as an insoluble solid called crud. Part of the radionuclides in the reactor water is removed by the reactor purification system. However, the radionuclide that has not been removed accumulates on the surface of the structural member in contact with the reactor water while circulating in the recirculation system together with the reactor water. As a result, radiation is emitted from the surface of the structural member, which causes radiation exposure of workers during regular inspection work. The exposure dose of the employee is managed so as not to exceed the specified value for each person. However, in recent years, with this specified value lowered, it is necessary to make the exposure dose of each person as low as economically possible.

A chemical decontamination method has been proposed, which removes an oxide film containing radionuclides such as cobalt 60 and cobalt 58 formed on the surface of a structural member of a nuclear power plant that has undergone operation, for example, of piping by dissolution using chemicals (Japanese Patent Laid-open No. 2000-105295).

Further, various methods for reducing deposition of radionuclides on piping are studied. For example, Japanese Patent Laid-open No. 2006-38483 proposes a method for suppressing deposition of radionuclides on the surface of the structural member after the operation of the nuclear power plant by, after chemical decontamination, forming a magnetite film that is a kind of ferrite film on the surface of the nuclear power plant structural member that contacts the reactor water. Furthermore, Japanese Patent Laid-open No. 2006-38483 describes, after starting up a nuclear power plant, contacting the reactor water in which a noble metal is injected with the magnetite film to deposit the noble metal on the magnetite film.

Japanese Patent Laid-open No. 2007-182604 describes forming a nickel ferrite film on the surface of a carbon steel structural member in a nuclear power plant instead of the magnetite film during the operation shut-down of the nuclear power plant. By forming the nickel ferrite film, corrosion of the carbon steel structural member is suppressed, and deposition of the radionuclide on the structural member is suppressed.

Furthermore, a method of forming a nickel metal film on the surface of the carbon steel member, forming a nickel ferrite film on the surface of the nickel metal film by using a film forming solution containing nickel ions, iron (II) ions, an oxidant and a pH adjusting agent, having a pH in the range of 5.5 to 9.0, and a temperature in the range of 60° C. to 100° C., and then converting the nickel metal film into a nickel ferrite film with high temperature water has been proposed (For example, Japanese Patent Laid-open No. 2011-32551).

Japanese Patent Laid-open No. 2010-127788 describes, when forming a ferrite film on the surface of nuclear power plant components that contact reactor water, measuring the amount of this ferrite film formed with a quartz crystal electrode device, and determining the end of the formation of the ferrite film on the surface of the components based on the measured formation amount of the ferrite film.

Japanese Patent Laid-open No. 2018-48831 discloses contacting a film forming solution containing nickel ions with the surface of a carbon steel member that contacts reactor water to form a nickel metal film on the surface, and depositing a noble metal on the surface of the nickel metal film and contacting the oxygen-containing water of 200° C. or more with the surface of the nickel metal film on which the noble metal is deposited, so that the nickel metal film is converted into a stable nickel ferrite film (a nickel ferrite film in which x is 0 in Ni_(1−x)Fe_(2+x)O₄) that covers the surface of the carbon steel member and that does not dissolve by the action of the noble metal.

Japanese Patent Laid-open No. 2018-48831 discloses converting a nickel metal film, which is formed on a surface of a carbon steel member that contacts the reactor water to cover the surface and on which a noble metal is deposited, into a stable nickel ferrite film (for example, NiFe₂O₄ film) that covers the surface of the carbon steel member and that does not dissolve by the action of noble metal, thus suppressing deposition of the radionuclides on the carbon steel member of the nuclear power plant for a long period of time.

However, rather than the method for depositing a noble metal on the carbon steel member of the nuclear power plant described in Japanese Patent Laid-open No. 2018-48831, it is more desirable to shorten the time required for forming the nickel metal film on the surface of the carbon steel member.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method for depositing a noble metal on a carbon steel member of a nuclear power plant and a method for suppressing the radionuclide deposition on a carbon steel member of a nuclear power plant, which can reduce the time required for forming a nickel metal film on the surface of the carbon steel member.

A feature of the present invention that achieves the object described above resides in a method for depositing a noble metal on a carbon steel member of a nuclear power plant (hereinafter referred to as a first noble metal deposition method), including: contacting a film forming solution containing nickel ions and a surface purification agent and having a pH within a range of 1.8 or more to 2.5 or less with a first surface of a carbon steel member of a nuclear power plant that contacts reactor water, and forming a nickel metal film on the first surface and depositing a noble metal on a second surface of the formed nickel metal film, in which the forming the nickel metal film and the depositing the noble metal are performed after shut-down of the nuclear power plant and before start-up of the nuclear power plant.

The object described above can be achieved by a method for depositing a noble metal on a carbon steel member of a nuclear power plant (hereinafter referred to as a second noble metal deposition method), including: contacting a surface purification agent solution containing a surface purification agent and having a pH within a range of 1.8 or more to 2.5 or less with a first surface of a carbon steel member of a nuclear power plant that contacts reactor water; after a set time is elapsed since the surface purification agent solution is contacted with the first surface of the carbon steel member, injecting nickel ions into the surface purification agent solution to produce a film forming solution; contacting the film forming solution with the first surface of the carbon steel member to form a nickel metal film on the first surface; and depositing a noble metal on a second surface of the formed nickel metal film, in which the forming the nickel metal film and the depositing the noble metal are performed after shut-down of the nuclear power plant and before start-up of the nuclear power plant.

According to the first noble metal deposition method and the second noble metal deposition method, dissolution of iron ions from the carbon steel member is increased by the action of the surface purification agent, and the amount of electrons generated with the increased dissolution of iron ions is also increased, and accordingly, the substitution reaction between the iron (II) ions and the nickel ions contained in the film forming solution is promoted, and the amount of nickel ions incorporated into the surface of the carbon steel member is increased. The nickel ions incorporated into the surface of the carbon steel member are reduced by the electrons mentioned above into a nickel metal. For this reason, the formation of the nickel metal film on the surface of the carbon steel member is promoted, and the time required for forming the nickel metal film is remarkably shortened.

The object described above can also be achieved by a method for suppressing the deposition of radionuclides on a carbon steel member of a nuclear power plant, which, after the first noble metal deposition method or the second noble metal deposition method is performed to form the nickel metal film on the surface of the carbon steel member, contacts oxygen-containing water of a temperature range of 130° C. or more and 330° C. or less with the surface of the nickel metal film on which a noble metal is deposited.

Since the nickel metal film can be converted into a stable nickel ferrite film by contacting oxygen-containing water having a temperature range of 130° C. or more and 330° C. or less with the surface of the nickel metal film on which the noble metal is deposited, the surface of the carbon steel member is further covered with the stable nickel ferrite film, and deposition of radionuclides on the carbon steel member can further be suppressed over a long period of time.

According to the present invention, the time required for forming a nickel metal film on the surface of a carbon steel member of a nuclear power plant can be shortened.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing a procedure of a method for depositing a noble metal on a carbon steel member of a nuclear power plant according to a first embodiment applied to a purification system piping of a boiling water type nuclear power plant, which is a preferred embodiment of the present invention.

FIG. 2 is an explanatory view showing a state in which a film forming apparatus used when carrying out the method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the first embodiment is connected to a purification system piping of a boiling water type nuclear power plant.

FIG. 3 is a detailed configuration view showing the film forming apparatus shown in FIG. 2.

FIG. 4 is a cross-sectional view of a purification system piping of a boiling water type nuclear power plant before the method for depositing a noble metal on a carbon steel member of a nuclear power plant shown in FIG. 1 is started.

FIG. 5 is an explanatory view showing a state in which Fe²⁺ is dissolved from a carbon steel member when an aqueous solution containing a surface purification agent is contacted with the surface of the carbon steel member in the method for depositing a noble metal on a carbon steel member of the nuclear power plant shown in FIG. 1.

FIG. 6 is an explanatory view showing a state in which a nickel metal film is formed on an inner surface of a purification system piping according to a method for forming a nickel metal film on a carbon steel member of the nuclear power plant shown in FIG. 1.

FIG. 7 is an explanatory view showing a state in which noble metal is deposited on a surface of a nickel metal film formed on an inner surface of the purification system piping according to a method of depositing a nickel metal film and noble metal on the carbon steel member of the nuclear power plant shown in FIG. 1.

FIG. 8 is an explanatory view showing a formation amount of nickel metal film (thickness of nickel metal film) formed on each of a carbon steel specimen of which surface was not treated with a surface purification agent and a carbon steel specimen of which surface was treated with a surface purification agent.

FIG. 9 is a flowchart showing a procedure of a method for depositing a noble metal on a carbon steel member of a nuclear power plant according to a second embodiment applied to a purification system piping of a boiling water type nuclear power plant, which is another preferred embodiment of the present invention.

FIG. 10 is a detailed configuration view of a film forming apparatus used in the method for depositing a noble metal on a carbon steel member of a nuclear power plant shown in FIG. 9.

FIG. 11 is a flowchart showing a procedure of a method for suppressing deposition of radionuclides on a carbon steel member of a nuclear power plant according to a third embodiment applied to a purification system piping of a boiling water type nuclear power plant, which is another preferred embodiment of the present invention.

FIG. 12 is an explanatory view of the method for suppressing deposition of radionuclides on a carbon steel member of the nuclear power plant shown in FIG. 11, showing a state in which oxygen-containing reactor water having a temperature range of 130° C. or more and 280° C. or less is contacted with a nickel metal film which is formed on an inner surface of a purification system piping and which has platinum deposited thereon.

FIG. 13 is an explanatory view of the method for suppressing deposition of radionuclides on a carbon steel member of a nuclear power plant shown in FIG. 11, showing a state in which oxygen contained in the reactor water at a temperature range of 130° C. or more and 280° C. or less, and Fe²⁺ in the purification system piping are migrated to a nickel metal film which is formed on an inner surface of a purification system piping and which has platinum deposited thereon.

FIG. 14 is an explanatory view of the method for suppressing deposition of radionuclides on a carbon steel member of a nuclear power plant shown in FIG. 11, showing a state in which a nickel metal film, which is formed on an inner surface of a purification system piping and which has platinum deposited thereon, is converted into a stable nickel ferrite film.

DESCRIPTION OF EMBODIMENTS

The inventors have studied various measures for shortening the time required for forming a nickel metal film on the surface of a carbon steel member of a nuclear power plant in Japanese Patent Laid-open No. 2018-48831. The result of this investigation will be described below.

As a result of various studies, the inventors thought that the time required for forming the nickel metal film on the surface of the carbon steel member can be shortened by promoting, on the surface, the substitution reaction between the iron (II) ions (Fe²⁺) contained in the carbon steel member and the nickel ions contained in the film forming solution in contact with the surface. Furthermore, the inventors speculated that the promotion of the substitution reaction between iron (II) ions and nickel ions is caused by an increase in the amount of iron (II) ions dissolved from the carbon steel member. Therefore, the inventors attempted to increase the dissolution amount of iron (II) ions by contacting a surface purification agent such as a formic acid aqueous solution containing formic acid with the surface of the carbon steel member that underwent chemical decontamination.

Inventors performed the following experiment in order to confirm whether the time required for forming a nickel metal film on the surface of a carbon steel member can be shortened based on the above idea. In this experiment, a carbon steel specimen of which surface was subjected to chemical decontamination, specifically, to a reductive decontamination using oxalic acid, was used.

In the experiment, a nickel metal film is formed on the surface of one carbon steel specimen (hereinafter referred to as “specimen A”) according to the method described in Japanese Patent Laid-open No. 2018-48831. Specifically, 90° C. water into which nickel formate was injected, that is, 1L (liter) of a 90° C. film forming aqueous solution containing 400 ppm of nickel ions and 800 ppm of formic acid is filled in the container, and the specimen A is immersed in the film forming aqueous solution in the container (for example, a beaker) and contacted with the film forming aqueous solution for 4 hours. When 4 hours elapse, the specimen A is taken out from the film forming aqueous solution. A nickel metal film is formed on the surface of the specimen A by contacting with the film forming aqueous solution.

In addition, another carbon steel specimen (hereinafter referred to as “specimen B”) subjected to reductive decontamination is immersed into a 90° C. formic acid aqueous solution (surface purification agent aqueous solution) containing 30000 ppm of formic acid filled in a container, which is a surface purification agent, and the formic acid aqueous solution is contacted with the surface of the specimen B for 1 hour. When 1 hour elapses, a 40 cc of nickel formate aqueous solution containing 400 ppm of nickel ions and 800 ppm of formic acid is injected into the formic acid aqueous solution that is the surface purification agent aqueous solution in the container. The specimen B is immersed in the film forming aqueous solution obtained by injecting the nickel formate aqueous solution into the surface purification agent aqueous solution in a container, and contacted with the film forming aqueous solution for 1 hour. A nickel metal film is formed on the surface of the specimen B during 1 hour of contact with the film forming aqueous solution.

The surface purification agent described above is used to remove iron hydroxide precipitated on the surface (specifically, on inner surface of a carbon steel piping (for example, inner surface of the purification system piping of the reactor purification system)) of a carbon steel member of a nuclear power plant contacted with water after completion of chemical decontamination, specifically, reductive decontamination, and also to promote the dissolution of iron (II) ions from the carbon steel member. The surface purification agent is an organic substance that can be decomposed, and any of formic acid, malonic acid, and ascorbic acid may be used.

As the 90° C. formic acid aqueous solution (surface purification agent aqueous solution) containing 30000 ppm of formic acid is contacted with the surface of the specimen B that underwent reductive decontamination, by the action of 30000 ppm of formic acid, the amount of iron (II) ions dissolved from the carbon steel specimen B into the surface purification agent aqueous solution is significantly increased in 1 hour. As the dissolution amount of iron (II) ions increases, the amount of electrons generated also increases. The dissolved iron (II) ions and the generated electrons are accumulated in the surface purification agent aqueous solution. Due to the action of 30000 ppm of formic acid contained in the film forming aqueous solution obtained by injecting the nickel formate aqueous solution containing nickel ions and formic acid at the above concentrations into the surface purification agent aqueous solution, a large amount of iron (II) ions is dissolved from the specimen B, and this promotes the substitution reaction between the iron (II) ions and nickel ions contained in the film forming aqueous solution, and the amount of nickel ions incorporated into the surface of the specimen B increases. Even after the nickel formate aqueous solution is injected, electrons corresponding to the dissolution amount of iron (II) ions are generated. The nickel ions incorporated into the surface of the specimen B are reduced by the electrons into nickel metal. For this reason, a nickel metal film is formed on the surface of the specimen B, and the entire surface is eventually covered with the nickel metal film. The action of 30000 ppm of formic acid increases the dissolution amount of iron (II) ions from the specimen B and promotes the substitution reaction between iron (II) ions and nickel ions, so that the time required until the entire surface of the specimen B is covered with the nickel metal film is significantly shortened.

When the entire surface of the specimen B is covered with the nickel metal film, dissolution of iron (II) ions from the specimen B stops. However, when the entire surface of the specimen B is covered with the nickel metal film, since a large amount of electrons, which is equivalent to the amount of electrons generated before injecting the nickel formate aqueous solution into the surface purification agent aqueous solution containing 30000 ppm of formic acid, is accumulated in the surface purification agent aqueous solution, even after the injection of the nickel formate aqueous solution, the nickel ions contained in the obtained film forming aqueous solution and adsorbed on the surface of the nickel metal film are reduced by the accumulated electrons into nickel metal. For this reason, without using a reducing agent, the thickness of the nickel metal film formed on the surface of the specimen B is increased, and the time required for forming the nickel metal film on the surface of the specimen B is also shortened.

The time from when the specimen B is contacted with the film forming aqueous solution formed by the injection of the nickel formate aqueous solution to when the formation of the nickel metal film on the surface of the specimen B is completed (that is, sum of the time required from when the specimen B is contacted with the film forming aqueous solution obtained by injecting the nickel formate aqueous solution to when the nickel metal film is formed on the surface of the specimen B, and the time required from when the nickel metal film is formed on the entire surface of the specimen B to when the formation of the nickel metal film on the surface of the specimen B is completed) is the 1 hour described above.

The inventors measured the formation amount of nickel metal film formed on the surfaces of the specimens A and B, respectively. The measurement results are shown in FIG. 8. The formation amount of the nickel metal film formed on the surface of the specimen B is 18 times the formation amount of the nickel metal film formed on the surface of the specimen A. That is, the formation amount of the nickel metal film, which is formed on the surface of the specimen B by contacting the surface purification agent aqueous solution having a formic acid concentration of 30000 ppm with the surface and then contacting a film forming aqueous solution obtained by injecting nickel formate into the surface purification agent aqueous solution with the surface, is 18 times the formation amount of the nickel metal film formed on the surface of the specimen A according to the method described in Japanese Patent Laid-open No. 2018-48831 (without using a surface purification agent aqueous solution containing formic acid at a concentration within the range of 2500 ppm or more to 120000 ppm or less).

The formic acid concentration of the surface purification agent aqueous solution is preferably within the range of 2500 ppm to 120000 ppm (2500 ppm or more to 120000 ppm or less). When the formic acid concentration is 30000 ppm, the formation amount of nickel metal film formed on the surface of the carbon steel member is the largest. When the formic acid concentration is less than 2500 ppm and greater than 120000 ppm, the formation amount of nickel metal film is less than half the formation amount of nickel metal film formed when it is 30000 ppm. For this reason, preferably, the formic acid concentration of the surface purification agent aqueous solution is set to a concentration within the range of 2500 ppm or more to 120000 ppm or less.

The pH of the surface purification agent aqueous solution is set to a pH within the range of 1.8 or more to 2.5 or less. When the pH of the surface purification agent aqueous solution is less than 1.8, nickel is present as nickel ions, and a nickel metal film is not formed on the surface of the carbon steel member. When the pH of the surface purification agent aqueous solution is greater than 2.5, dissolution of iron (II) ions is delayed. For this reason, the pH of the surface purification agent aqueous solution is preferably set to a pH within the range of 1.8 or more to 2.5 or less. When the formic acid concentration of the surface purification agent aqueous solution is 120000 ppm, the pH of the surface purification agent aqueous solution is 1.8, and when the formic acid concentration of the surface purification agent aqueous solution is 2500 ppm, the pH of the surface purification agent aqueous solution is 2.5. The pH of the surface purification agent aqueous solution having a formic acid concentration of 30000 ppm is 2.15.

Even when either malonic acid or ascorbic acid is used in place of formic acid, the pH of the surface purification agent aqueous solution is set to a pH within the range of 1.8 or more to 2.5 or less. That is, a surface purification agent aqueous solution which is a malonic acid aqueous solution having a pH within a range of 1.8 or more to 2.5 or less contains malonic acid at a concentration within a range of or less than a concentration that makes the pH of the aqueous solution 2.5, to or more than a concentration that makes the pH of the aqueous solution 1.8. Moreover, the surface purification agent aqueous solution which is an ascorbic acid aqueous solution having a pH within a range of 1.8 or more to 2.5 or less contains ascorbic acid at a concentration within a range of or less than a concentration that makes the pH of the aqueous solution 2.5, to or more than a concentration that makes the pH of the aqueous solution 1.8. As a result, the pH of the surface purification agent aqueous solution containing any one of formic acid, malonic acid and ascorbic acid as the surface purification agent is a pH within the range of 1.8 or more to 2.5 or less.

Although it is described above that, when a certain amount of time elapses after the surface purification agent aqueous solution containing 30000 ppm of formic acid is contacted with the carbon steel member, a nickel formate aqueous solution is injected into the surface purification agent aqueous solution to form a film forming aqueous solution and the film forming aqueous solution is contacted with the carbon steel member, the film forming aqueous solution obtained by mixing the surface purification agent aqueous solution and the nickel formate aqueous solution may be contacted with the carbon steel member, that is, both formic acid (surface purification agent) and nickel ions may be contacted with the carbon steel member at the same time. However, in this case, when the surface of the carbon steel member is covered with the nickel metal film, dissolution of iron (II) ions from the specimen B stops, and the generation of electrons stops as the dissolution of iron (II) ions stops. For this reason, when formic acid (surface purification agent) and nickel ions are contacted with the carbon steel member at the same time, the formation amount of nickel metal film formed on the surface of the carbon steel member is smaller than when the surface purification agent aqueous solution containing formic acid (surface purification agent) is contacted with the carbon steel member, and after a certain period of time, a film forming aqueous solution obtained by injecting a nickel formate aqueous solution into the surface purification agent aqueous solution is contacted with the carbon steel member. However, even when formic acid (surface purification agent) and nickel ions are contacted with the carbon steel member at the same time, 30000 ppm of formic acid (surface purification agent) is contacted with the carbon steel member and the dissolution amount of iron (II) ions from the specimen B increases, and the generation amount of electrons having a reducing action is also increased as compared with the case in which the surface purification agent aqueous solution is not contacted with the carbon steel member. For this reason, the time required for the entire surface of the carbon steel member to be covered with the nickel metal film is shorter than the time required for the entire surface of the carbon steel member to be covered with the nickel metal film by contacting a film forming aqueous solution containing 400 ppm of nickel ions and 800 ppm of formic acid with the carbon steel member as in Japanese Patent Laid-open No. 2018-48831.

In addition, since the generation amount of electrons increases, even when the entire surface of the carbon steel member is covered with the nickel metal film, electrons remain in the film forming aqueous solution such that nickel ions deposited on the surface of the nickel metal film covering the entire surface of the carbon steel member are converted into nickel metal by the reducing action of the remaining electrons. Therefore, when the formation of the nickel metal film is completed, the formation amount (thickness) of the nickel metal film on the surface of the carbon steel member is increased from the formation amount of the nickel metal film at the time when the entire surface of the carbon steel member is covered with the nickel metal film.

Even when either malonic acid or ascorbic acid is used in place of formic acid, the concentration of malonic acid or ascorbic acid in the surface purification agent aqueous solution is set to a concentration within the range of 2500 ppm or more to 120000 ppm or less.

After the formation of the nickel metal film on the surface of the carbon steel member is completed as described above, an aqueous solution containing noble metal ions (for example, platinum ions) and a reducing agent (for example, hydrazine) is contacted with the surface of the nickel metal film, and the noble metal (for example, platinum) is deposited on the surface of the nickel metal film. For a reducing agent used when depositing a noble metal on the surface of a carbon steel member, any of hydrazine derivatives such as hydrazine, formhydrazine, hydrazinecarboxamide and carbohydrazide, and hydroxylamine may be used. Thus, the method for depositing a noble metal on a carbon steel member of a nuclear power plant is completed.

Next, an outline of the method for suppressing the deposition of radionuclides on a carbon steel member of a nuclear power plant will be described below. After the deposition of the noble metal on the carbon steel member is completed, an operation for suppressing the deposition of the radionuclide on the surface of the carbon steel member, that is, converting the nickel metal film into a stable ferrite film will be described.

As described in Japanese Patent Laid-open No. 2018-48831, when a magnetite film, which is a kind of ferrite film, is formed on the surface of a structural member of a nuclear plant according to the method described in Japanese Patent Laid-open No. 2006-38483 and Japanese Patent Laid-open No. 2012-247322 described above, there is a possibility that the formed magnetite film is dissolved into the reactor water by the action of the deposited noble metal, and at the end of the operation cycle, the ferrite film may disappear and the effect of suppressing the deposition of radionuclides by the ferrite film may be lost. When the ferrite film disappears, it is necessary to form the ferrite film again on the surface of the carbon steel member after shut-down of the nuclear power plant in this operation cycle. The reason for the ferrite film formed on the surface of the carbon steel member being dissolved by the action of the deposited noble metal is described in paragraph 0036 of Japanese Patent Laid-open No. 2018-48831.

Further, as described in Japanese Patent Laid-open No. 2011-32551, oxygen-containing water of 150° C. or more is contacted with the nickel ferrite film that covers the nickel metal film formed on the surface of the carbon steel member of the BWR plant and contains nickel ferrite having a high iron content, so that when the nickel metal film is converted into a nickel ferrite film, the nickel ferrite film converted from the nickel metal film becomes an unstable nickel ferrite film (for example, Ni_(0.7)Fe_(2.3)O₄ film). The conversion of the nickel metal film into an unstable nickel ferrite film occurs because the amount of iron supplied to the nickel metal film is increased and the amount of nickel is insufficient when the nickel metal film is converted into a nickel ferrite film.

The nickel ferrite film that originally covered the nickel metal film is contacted with the high temperature water and then reacts with the nickel metal migrated from the nickel metal film to be a Ni_(0.7)Fe_(2.3)O₄ film. The Ni content of the nickel ferrite of the original nickel ferrite film is lower than that of Ni_(0.7)Fe_(2.3)O₄, and the original nickel ferrite film is a nickel ferrite film that is unstable in a reducing environment. The unstable nickel ferrite is a nickel ferrite satisfying 0.3≤x<1.0 in Ni_(1−x)Fe_(2+x)O₄, for example, Ni_(0.7)Fe_(2.3)O₄.

For this reason, in Japanese Patent Laid-open No. 2011-32551, as in Japanese Patent Laid-open No. 2006-38483, the nickel ferrite film is dissolved into the reactor water due to the deposition of the noble metal injected to the surface of the unstable nickel film during the operation of the BWR plant. Eventually, at the end of the operation cycle, there is a possibility that the unstable nickel ferrite film disappears, and the carbon steel member may be exposed and contact the reactor water.

Meanwhile, at the time of deposition of the noble metal on the surface of the carbon steel member, when Fe²⁺ is dissolved from the carbon steel member, the noble metal cannot be deposited on the surface of the carbon steel member. In order to prevent the dissolution of Fe²⁺ from the carbon steel member, and perform the deposition of the noble metal on the carbon steel member in a short time and increase the amount of the deposition, as described in Japanese Patent Laid-open No. 2018-48831, it is preferable to cover the surface of the carbon steel member with a nickel metal film.

The inventors aimed to realize a long-term suppression of the deposition of radionuclides on a carbon steel member of a nuclear power plant by, instead of forming Ni_(0.7)Fe_(2.3)O₄ film that is unstable at a low temperature range of 60° C. to 100° C. on the surface of a carbon steel member, forming a stable nickel ferrite film on the surface of the carbon steel member that does not dissolve with the deposited novel metal. In view of this, the inventors have made various studies on the use of the nickel metal film formed on the surface of the carbon steel member for forming a stable nickel ferrite film in order to achieve longer-term suppression of radionuclide deposition. As a result, the inventors discovered that, by contacting oxygen-containing water (for example, reactor water) at a temperature range of 130° C. or more and 330° C. or less with the nickel metal film formed on the surface of the carbon steel member to form a high temperature environment at a temperature range of 130° C. or more and 330° C. or less for the carbon steel member and the nickel metal film, and by the action of a noble metal deposited on the surface of the nickel metal film, the nickel metal contained in the nickel metal film formed on the surface of the carbon steel member is converted into a stable nickel ferrite. This stable nickel ferrite is nickel ferrite satisfying 0≤x<0.3 in Ni_(1−x)Fe_(2+x)O₄, for example, nickel ferrite (NiFe₂O₄) in which x is 0 in Ni_(1−x)Fe_(2+x)O₄. Eventually, the nickel metal film formed on the surface of the carbon steel member is a stable nickel ferrite film that covers the surface of the carbon steel member and that does not dissolve by the action of the deposited noble metal (for example, nickel ferrite film (NiFe₂O₄ film) in which x is 0 in Ni_(1−x)Fe_(2+x)O₄).

For the noble metal to be deposited on the surface of the nickel metal film formed on the surface of the carbon steel member, any of platinum, palladium, rhodium, ruthenium, osmium, and iridium may be used.

The reason for the nickel metal film, which is formed on the surface of a carbon steel member and deposited with a noble metal (for example, platinum), being converted into a stable nickel ferrite film (NiFe₂O₄ film) covering the surface of the carbon steel member upon being contacted with oxygen-containing water at a temperature range of 130° C. or more (preferably 130° C. or more and 330° C. or less) will be described below. When water at 130° C. or more is contacted with the nickel metal film on the carbon steel member, the nickel metal film and the carbon steel member are heated to 130° C. or more. Oxygen contained in the water is migrated into the nickel metal film, and Fe contained in the carbon steel member is converted to Fe²⁺ and migrated into the nickel metal film. In a high temperature environment of 130° C. or more and, for example, due to the action of platinum deposited on the nickel metal film, nickel in the nickel metal film reacts with oxygen and Fe²⁺ migrated into the nickel metal film, and a nickel ferrite in which x is 0 in Ni_(1−x)Fe_(2+x)O₄, is produced, for example. At this time, whether nickel or iron is incorporated easily into the ferrite structure depends on the noble metal such that, when the noble metal is present, nickel is more easily incorporated than iron and a stable nickel ferrite (NiFe₂O₄) in which x is 0 in Ni_(1−x)Fe_(2+x)O₄ is produced. This stable nickel ferrite film covers the surface of the carbon steel member.

In the nickel ferrite generated as described above in which x is 0 in Ni_(1−x)Fe_(2+x)O₄, crystal grows significantly, and even when the noble metal is deposited, unlike Ni_(0.7)Fe_(2.3)O₄ film, it remains stable without dissolving into water and also acts to suppress the deposition of radionuclide on the carbon steel of the base material. As described above, the stable nickel ferrite film produced under the high temperature environment of 130° C. or more and by the action of platinum can suppress the deposition of radionuclides on the carbon steel member over a longer period of time than the Ni_(0.7)Fe_(2.3)O₄ film that is produced at a low temperature range of 60° C. to 100° C.

When the temperature of the oxygen-containing water contacted with the nickel metal film is less than 130° C., the nickel metal film is not converted into a stable nickel ferrite film (NiFe₂O₄ film). In order to convert the nickel metal film into a stable nickel ferrite film that does not dissolve by the action of the noble metal, it is necessary to set the temperature of oxygen-containing water which is to be contacted with the nickel metal film to a temperature range of 130° C. or more (130° C. or more and 330° C. or less).

Embodiments of the present invention reflecting the above results of the investigation will be described below.

First Embodiment

A method for depositing a noble metal on a carbon steel member of a nuclear power plant according to a first embodiment, which is a preferred embodiment of the present invention, will be described with references to FIGS. 1, 2, and 3. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the present embodiment is applied to a carbon steel purification system piping (carbon steel member) of a boiling water type nuclear power plant (BWR plant).

A schematic configuration of the BWR plant will be described with reference to FIG. 2. The BWR plant 1 includes a nuclear reactor 2, a turbine 9, a condenser 10, a recirculation system, a nuclear reactor purification system, a feedwater system, and the like. The reactor 2 includes a reactor pressure vessel (hereinafter referred to as RPV) 3 in which a reactor core 4 is built, and includes a plurality of jet pumps 5 installed in an annular downcomer formed between an outer surface of a reactor core shroud (not shown) surrounding the reactor core 4 in the RPV 3 and an inner surface of the RPV 3. A large number of fuel assemblies (not shown) are loaded in the reactor core 4. The fuel assembly includes a plurality of fuel rods filled with a plurality of fuel pellets made of nuclear fuel material. The recirculation system includes a stainless steel recirculation system piping 6 and a recirculation pump 7 installed in the recirculation system piping 6. In the feedwater system, a condensate pump 12, a condensate purification device (for example, a condensate demineralizer) 13, a low pressure feedwater heater 14, a feedwater pump 15, and a high pressure feedwater heater 16 are installed in this order from the condenser 10 toward the RPV 3 in a carbon steel feedwater piping 11 that connects the condenser 10 and the RPV 3. The reactor purification system includes a purification pump 19, a regenerative heat exchanger 20, a non-regenerative heat exchanger 21, and a reactor water purification device 22, which are installed in this order in a carbon steel purification system piping 18 that connects the recirculation system piping 6 and the feedwater piping 11. The purification system piping 18 is connected to the recirculation system piping 6 upstream of the recirculation pump 7. The nuclear reactor 2 is installed in a nuclear reactor containment vessel 26 disposed in a nuclear reactor building (not shown).

Cooling water (hereinafter referred to as “reactor water”) in the RPV 3 is pressurized by the recirculation pump 7 and jetted into the jet pump 5 through the recirculation system piping 6. The reactor water present around the nozzle of the jet pump 5 in the downcomer is also sucked into the jet pump 5 and supplied to the reactor core 4. The reactor water supplied to the reactor core 4 is heated by heat generated by fission of nuclear fuel material in the fuel rods. Part of the heated reactor water is converted into steam. This steam is guided from the RPV 3 through the main steam piping 8 to the turbine 9 to rotate the turbine 9. A generator (not shown) connected to the turbine 9 rotates to generate electric power. Steam discharged from the turbine 9 is condensed into water by the condenser 10. This water is supplied, as feedwater, into the RPV 3 through the feedwater piping 11. The feedwater flowing through the feedwater piping 11 is pressurized by the condensate pump 12, removed of the impurities at the condensate purification device 13, and further pressurized by the feedwater pump 15. The feedwater is heated by the low pressure feedwater heater 14 and the high pressure feedwater heater 16 and guided into the RPV 3. The extraction steam extracted from the turbine 9 by a extraction piping 17 is supplied to the low pressure feedwater heater 14 and the high pressure feedwater heater 16 respectively to be a heating source of the feedwater. The extracted steam is converted into drain water in the low pressure feedwater heater 14 and the high pressure feedwater heater 16, and this drain water is guided to the condenser 10 through the drain water piping 27.

A part of the reactor water flowing in the recirculation system piping 6 flows into the purification system piping 18 by the drive of the purification system pump 19 and is cooled by the regenerative heat exchanger 20 and the non-regenerative heat exchanger 21, and then is purified by the reactor water purification device 22. The purified reactor water is heated by the regenerative heat exchanger 20 and returned to the RPV 3 through the purification system piping 18 and the feedwater piping 11.

In the method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the present embodiment, a film forming apparatus 30 is used. The film forming apparatus 30 is connected to the purification system piping 18 as shown in FIG. 2.

A detailed configuration of the film forming apparatus 30 will be described with reference to FIG. 3.

The film forming apparatus 30 includes a surge tank 31, a circulation pump 32 and 33, a circulation piping 34, a nickel ion injection device 35, a reducing agent injection device 40, a platinum ion injection device (noble metal ion injection device) 45, a heater 51, a cooler 52, a cation exchange resin tower 53, a mixed bed resin tower 54, a decomposition device 55, an oxidant supply device 56, an ejector 61, and a formic acid injection device 78.

An on-off valve 62, the circulation pump 33, valves 63, 66, 68 and 73, the surge tank 31, the circulation pump 32, a valve 76 and an on-off valve 77 are provided in the circulation piping 34 in this order from upstream. A piping 65 that bypasses the valve 63 is connected to the circulation piping 34, and a valve 64 and a filter 50 are installed in the piping 65. The cooler 52 and a valve 67 are installed in a piping 68A that bypasses the valve 66 and is connected to the circulation piping 34 at both ends. The cation exchange resin tower 53 and a valve 69 are installed in a piping 70 that is connected to the circulation piping 34 at both ends and bypasses the valve 68. The mixed bed resin tower 54 and a valve 71 are installed in a piping 72 that is connected to the piping 70 at both ends and bypasses the cation exchange resin tower 53 and the valve 69. The cation exchange resin tower 53 is filled with a cation exchange resin, and the mixed bed resin tower 54 is filled with a cation exchange resin and an anion exchange resin.

A piping 75, installed with a valve 74 and the decomposition device 55 located downstream of the valve 74, bypasses a valve 73 and is connected to the circulation piping 34. The decomposition device 55 is filled with an activated carbon catalyst in which ruthenium is impregnated on the surface of the activated carbon, for example. The surge tank 31 is installed in the circulation piping 34 between the valve 73 and the circulation pump 32. The heater 51 is disposed in the surge tank 31. A piping 79A provided with a valve 78A and the ejector 61 is connected to the circulation piping 34 between the valve 76 and the circulation pump 32, and further connected to the surge tank 31. The ejector 61 is provided with a hopper (not shown) for supplying oxalic acid (reductive decontamination agent) used for reducing and dissolving contaminants on the inner surface of the recirculation system piping 6 into the surge tank 31.

The nickel ion injection device 35 includes a chemical tank 36, an injection pump 37, and an injection piping 38. The chemical tank 36 is connected to the circulation piping 34 by the injection piping 38 that includes the injection pump 37 and a valve 39. A nickel formate aqueous solution (aqueous solution containing nickel ions) prepared by dissolving nickel formate (Ni(HCOO)₂.2H₂O) in water is filled in the chemical tank 36.

The platinum ion injection device (noble metal ion injection device) 45 includes a chemical tank 46, an injection pump 47, and an injection piping 48. The chemical tank 46 is connected to the circulation piping 34 by the injection piping 48 that includes the injection pump 47 and a valve 49. An aqueous solution (for example, sodium hexahydroxoplatinate hydrate solution) containing platinum ions prepared by dissolving a platinum complex (for example, sodium hexahydroxoplatinate hydrate (Na₂[Pt(OH)₆].nH₂O)) in water is filled in the chemical tank 46. An aqueous solution containing platinum ions is a kind of aqueous solution containing noble metal ions. For an aqueous solution containing noble metal ions, an aqueous solution containing any ion of palladium, rhodium, ruthenium, osmium and iridium may be used in addition to the aqueous solution containing platinum ions.

The reducing agent injection device 40 includes a chemical tank 41, an injection pump 42, and an injection piping 43. The chemical tank 41 is connected to the circulation piping 34 by the injection piping 43 that includes the injection pump 42 and a valve 44. An aqueous solution of hydrazine which is a reducing agent is filled inside the chemical tank 41.

The formic acid injection device 78 includes a chemical tank 79, an injection pump 80, and an injection piping 81. The chemical tank 79 is connected to the circulation piping 34 by the injection piping 81 that includes the injection pump 80 and a valve 82. An aqueous solution of formic acid (surface purification agent aqueous solution), which is a surface purification agent, is filled inside the chemical tank 79. As will be described below, the formic acid concentration of the formic acid aqueous solution in the chemical tank 79 is set high so that the formic acid concentration of the surface purification agent aqueous solution flowing in the purification system piping 18 is 30000 ppm.

The injection pipings 81, 38, 48 and 43 are connected in this order to the circulation piping 34 between the valve 76 and the on-off valve 77 from the valve 76 toward the on-off valve 77.

The oxidant supply device 56 includes a chemical tank 57, a supply pump 58, and a supply piping 59. The chemical tank 57 is connected to a piping 75 upstream of the valve 74 by the supply piping 59 that includes the supply pump 58 and a valve 60. A hydrogen peroxide which is an oxidant is filled inside of the chemical tank 57. For the oxidant, ozone or water with dissolved oxygen may be used.

A pH meter 83 is attached to the circulation piping 34 between the connection point between the injection piping 43 and the circulation piping 34 and the on-off valve 77.

The BWR plant 1 is shut-down after completing the operation in one operation cycle. After the shutdown, a part of the fuel assembly loaded in the reactor core 4 is taken out as a spent fuel assembly, and a new fuel assembly having a burnup of 0 GWd/t is loaded in the reactor core 4. After such a fuel change is completed, the BWR plant 1 is restarted for operation of a next operation cycle. Maintenance inspection of the BWR plant is performed using a period of time during which the BWR plant 1 is shut-down for the fuel change.

During the shut-down period of the BWR plant 1 as described above, a method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the present embodiment is implemented for a carbon steel piping system connected to the RPV 3 which is one of the carbon steel members in the BWR plant 1, for example, for a purification system piping 18. In this noble metal deposition method, the oxide film that surrounds the radionuclide formed on the inner surface of the purification system piping 18 contacted with the reactor water is removed by chemical decontamination, and then a nickel metal attachment process on the inner surface of the purification system piping 18, and deposition process of noble metal, for example, platinum, on the nickel metal are performed.

A method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the present embodiment will be described below based on the procedure shown in FIG. 1. In the method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the present embodiment, the formation of a nickel metal film on the surface of the carbon steel member and the deposition of the noble metal on the surface of the nickel metal film are performed using the film forming apparatus 30.

First, the film forming apparatus is connected to a carbon steel piping system that is target of film formation (step S1). When the BWR plant 1 is shut-down, for example, the bonnet of the valve 23 installed in the purification system piping 18 is opened to block the recirculation system piping 6 side. One end of the circulation piping 34 of the film forming apparatus 30 on the on-off valve 77 side is connected to the flange of the valve 23, and one end of the circulation piping 34 is connected to the purification system piping 18 upstream of the purification system pump 19. Meanwhile, the bonnet of the valve 25 installed in the purification system piping 18 is opened between the regenerative heat exchanger 20 and the non-regenerative heat exchanger 21 to block the non-regenerative heat exchanger 21 side. The other end of the circulation piping 34 on the on-off valve 62 side is connected to the flange of the valve 25, and the other end is connected to the purification system piping 18 downstream of the regenerative heat exchanger 20. Both ends of the circulation piping 34 are connected to the purification system piping 18 to form a closed loop including the purification system piping 18 and the circulation piping 34.

Chemical decontamination is performed for the carbon steel piping system that is target of film formation (step S2). In the BWR plant 1 that has experienced operation in the previous operation cycle, an oxide film containing a radionuclide is formed on the inner surface of the purification system piping 18 that contacts the reactor water flowing from the RPV 3. Before forming the nickel metal film on the inner surface of the purification system piping 18, it is preferable to remove the oxide film containing the radionuclide from the inner surface. In the present embodiment, when a film before conversion into nickel ferrite, which contains nickel metal and which is deposited with platinum, is formed on the inner surface of the purification system piping 18, in order to lower the dose rate of the purification system piping 18 in advance and to improve the adhesion between the film to be formed and the inner surface of the purification system piping 18, it is desirable to remove the oxide film that contains the radionuclide formed on the inner surface of the purification system piping 18. This removal of the oxide film also leads to an improvement in the adhesion between the nickel metal film and the inner surface of the purification system piping 18. In order to remove this oxide film, the chemical decontamination is performed, and in particular, the reductive decontamination using a reductive decontamination solution containing oxalic acid which is a reductive decontamination agent is performed with respect to the inner surface of the purification system piping 18.

At step S2, the chemical decontamination applied with respect to the inner surface of the purification system piping 18 is the known reductive decontamination described in Japanese Patent Laid-open No. 2000-105295. Oxidation decontamination is not performed with respect to the carbon steel purification system piping 18. Among the chemical decontamination processes, the reductive decontamination immediately before the process of depositing the nickel metal film and the noble metal is performed will be described.

First, with the on-off valve 62, the valves 63, 66, 68, 73 and 76, and the on-off valve 77 open respectively and with the other valves closed, the circulation pumps 32 and 33 are driven. Thereby, the water heated by the heater 51 in the surge tank 31 in the purification system piping 18 circulates in the closed loop formed by the circulation piping 34 and the purification system piping 18. The circulating water is adjusted to 90° C. by the heater 51. When the temperature of the water reaches 90° C., the valve 78A is opened to guide a part of the water flowing in the circulation piping 34 into the piping 79A. A predetermined amount of oxalic acid supplied from the hopper and ejector 61 into the piping 79A is guided into the surge tank 31 by the water flowing through the piping 79A. This oxalic acid is dissolved in water in the surge tank 31, and an oxalic acid aqueous solution (reductive decontamination solution) is produced in the surge tank 31.

The oxalic acid aqueous solution is discharged from the surge tank 31 to the circulation piping 34 by the drive of the circulation pump 32. With the valve 44 opened and by the drive of the injection pump 42, the hydrazine aqueous solution in the chemical tank 41 of the reducing agent injection device 40 is injected into the oxalic acid aqueous solution in the circulation piping 34 through the injection piping 43. The injection pump 42 (or the opening degree of the valve 44) is controlled based on the pH value of the oxalic acid aqueous solution measured by the pH meter 83 to adjust the injection amount of the hydrazine aqueous solution into the circulation piping 34, so that the pH of the oxalic acid aqueous solution supplied to the purification system piping 18 is adjusted to 2.5. In the present embodiment, hydrazine, which is a reducing agent used when depositing nickel metal on the inner surface of the purification system piping 18 and when depositing a noble metal such as platinum on the nickel metal film, is used as a pH adjusting agent for adjusting the pH of the oxalic acid aqueous solution in the reductive decontamination process.

An oxalic acid aqueous solution (reductive decontamination aqueous solution) containing 90° C. hydrazine having a pH of 2.5 is supplied from the circulation piping 34 to the purification system piping 18. The oxide film containing the radionuclide formed on the inner surface of the purification system piping 18 is dissolved by oxalic acid. The oxalic acid aqueous solution flows through the purification system piping 18 while dissolving the oxide film, passes through the purification system pump 19 and the regenerative heat exchanger 20, and is returned to the circulation piping 34. The oxalic acid aqueous solution returned to the circulation piping 34 is pressurized by the circulation pump 33 through the on-off valve 62 and reaches the surge tank 31. Thus, the oxalic acid aqueous solution containing hydrazine circulates in the closed loop including the circulation piping 34 and the purification system piping 18, and dissolves the oxide film formed on the inner surface of the purification system piping 18.

As the oxide film dissolves, the concentration of radionuclide and concentration of Fe of the oxalic acid aqueous solution containing hydrazine increase. In order to suppress these increase in concentration, the valve 69 is opened to reduce the opening degree of the valve 68, and a part of the oxalic acid aqueous solution containing hydrazine returned to the circulation piping 34 is guided to the cation exchange resin tower 53 by the piping 70. Metallic cations such as radionuclide and Fe contained in the oxalic acid aqueous solution are adsorbed and removed by the cation exchange resin in the cation exchange resin tower 53. The oxalic acid aqueous solution containing hydrazine discharged from the cation exchange resin tower 53 and the oxalic acid aqueous solution containing hydrazine passing through the valve 68 are supplied again from the circulation piping 34 to the purification system piping 18, and used for reductive decontamination of the purification system piping 18.

In the reductive decontamination with respect on the surface of a carbon steel member (for example, purification system piping 18) using oxalic acid, there are cases when iron oxalate (II) having low solubility is formed on the surface of the carbon steel member, and by this iron oxalate (II), dissolution by oxalic acid of the oxide film on the surface of the carbon steel member is suppressed. In this case, the valve 68 is fully opened, the valve 69 is closed, and the supply of the oxalic acid aqueous solution to the cation exchange resin tower 53 is stopped. Further, the valve 60 is opened to start the supply pump 58, and with the valve 74 closed, hydrogen peroxide in the chemical tank 57 is supplied to the oxalic acid aqueous solution containing hydrazine flowing through the circulation piping 34 through the supply piping 59 and the piping 75. The oxalic acid aqueous solution containing hydrogen peroxide and hydrazine is guided to the purification system piping 18. For this reason, Fe (II) contained in iron oxalate (II) formed on the inner surface of the purification system piping 18 is oxidized to Fe (III) by the action of the hydrogen peroxide, and the iron oxalate (II) is dissolved in the oxalic acid aqueous solution as an iron oxalate (III) complex. That is, iron oxalate (II), and hydrogen peroxide and oxalic acid contained in the oxalic acid aqueous solution cause the reaction shown in Equation (1) to produce iron oxalate (III) complex, water, and hydrogen ions.

2Fe(COO)₂+H₂O₂+2(COOH)₂→2Fe[(COO)₂]₂ ⁻+2H₂O+2H⁺  (1)

After it is confirmed that iron oxalate (II) formed on the inner surface of the purification system piping 18 was dissolved and hydrogen peroxide injected into the oxalic acid aqueous solution containing hydrazine disappeared by the reaction of the Equation (1), the valve 69 is opened to adjust the opening degree of the valve 68, and a part of the oxalic acid aqueous solution containing hydrazine flowing through the circulation piping 34 and passing through the valve 66 is supplied to the cation exchange resin tower 53 through the piping 70. Metallic cations such as radionuclides contained in the oxalic acid aqueous solution containing hydrazine are adsorbed and removed by the cation exchange resin in the cation exchange resin tower 53. The disappearance of hydrogen peroxide in the oxalic acid aqueous solution containing hydrazine may be confirmed by immersing a test paper that reacts with hydrogen peroxide in the oxalic acid aqueous solution sampled from the circulation piping 34 and observing the color appearing on the test paper.

When the dose rate of the reductive decontamination part of the purification system piping 18 is lowered to a set dose rate or when the reduction decontamination time of the purification system piping 18 reaches a predetermined time, oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed. That is, reductive decontamination decomposition process is performed. In addition, it may be confirmed from the dose rate calculated based on the output signal of the radiation detector that detects the radiation from the reductive decontamination location of the purification system piping 18 that the dose rate of the reductive decontamination location is lowered to the set dose rate.

Decomposition of oxalic acid and hydrazine contained in the oxalic acid aqueous solution is performed as follows. With the valve 74 opened and the opening degree of the valve 73 partially reduced, the oxalic acid aqueous solution containing hydrazine that has flowed through the cation exchange resin tower 53 and passed through the valve 69 is supplied to the decomposition device 55 through the piping 75. At this time, with the valve 60 opened and by the drive of the supply pump 58, hydrogen peroxide in the chemical tank 57 flows into the decomposition device 55 through the supply piping 59 and the piping 75. Oxalic acid and hydrazine contained in the oxalic acid aqueous solution are decomposed in the decomposition device 55 by the action of the activated carbon catalyst and the hydrogen peroxide supplied. The decomposition reaction of oxalic acid and hydrazine in the decomposition device 55 is expressed by the Equations (2) and (3).

(COOH)₂+H₂O₂→2CO₂+2H₂O  (2)

N₂H₄+2H₂P₂→N₂+4H₂O  (3)

The decomposition of the oxalic acid and hydrazine in the decomposition device 55 is performed while circulating the oxalic acid aqueous solution in the closed loop including the circulation piping 34 and the purification system piping 18. In order to prevent the supplied hydrogen peroxide from being completely consumed by the decomposition device 55 for the decomposition of oxalic acid and hydrazine and from flowing out of the decomposition device 55, the amount of hydrogen peroxide supplied from the chemical tank 57 to the decomposition device 55 is adjusted by controlling the rotational speed of the supply pump 58.

Also in the reductive decontamination decomposition process, there is a possibility that iron oxalate (II) may be formed on the inner surface of the purification system piping 18 that is a carbon steel member and contacted with the oxalic acid aqueous solution. Therefore, when the decomposition of oxalic acid and hydrazine contained in the oxalic acid aqueous solution is progressed to some extent, the rotational speed of the supply pump 58 is increased to increase the amount of hydrogen peroxide supplied from the chemical tank 57 to the decomposition device 55 so that hydrogen peroxide flows out from the decomposition device 55. At that time, the valve 69 is closed so that hydrogen peroxide does not flow into the cation exchange resin tower 53.

The oxalic acid aqueous solution containing hydrogen peroxide and hydrazine discharged from the decomposition device 55 is guided from the circulation piping 34 to the purification system piping 18. As described above, the iron oxalate (II) formed on the inner surface of the purification system piping 18 which is a carbon steel member is converted into the iron oxalate (III) complex by the action of hydrogen peroxide, and dissolved in an oxalic acid aqueous solution containing hydrazine. While the decomposition of the oxalic acid or the like in oxalic acid aqueous solution containing hydrazine is in process, there may be insufficient oxalic acid to convert Fe (II) contained in iron oxalate (II) into easily soluble Fe (III), and precipitation of Fe(OH)₃ on the inner surface of the circulation piping 34 is likely to occur. For this reason, in order to suppress the precipitation of Fe(OH)₃, formic acid is injected into the oxalic acid aqueous solution. As there is a high concentration of formic acid in the formic acid aqueous solution in the chemical tank 79 of the formic acid injection device 78, the injection of formic acid is performed by supplying formic acid from the hopper and ejector 61 to the oxalic acid aqueous solution and guiding it to the surge tank 31 with the valve 78A opened and the oxalic acid aqueous solution flowing in the piping 79A. The supplied formic acid is mixed with the oxalic acid aqueous solution in the circulation piping 34.

The supplied oxalic acid aqueous solution containing formic acid and hydrazine contains hydrogen peroxide discharged from the decomposition device 55 in addition to oxalic acid and hydrazine having a lowered concentration. The oxalic acid aqueous solution containing formic acid, hydrazine and hydrogen peroxide is supplied to the purification system piping 18. Hydrogen peroxide contained in the oxalic acid aqueous solution dissolves iron oxalate (II) precipitated on the inner surface of the purification system piping 18, and formic acid dissolves Fe(OH)₃. Since this oxalic acid aqueous solution circulates in a closed loop including the circulation piping 34 and the purification system piping 18, decomposition of oxalic acid and hydrazine is also continued in the decomposition device 55.

Next, in order to complete the decomposition process of oxalic acid and hydrazine, the hydrogen peroxide concentration of the oxalic acid aqueous solution flowing in the circulation piping 34 is lowered and the oxalic acid aqueous solution is supplied to the cation exchange resin tower 53. For this reason, the valve 60 is closed to stop the supply of hydrogen peroxide, and the injection pump 80 is stopped to close the valve 82 to stop the injection of formic acid, respectively. When the supply of hydrogen peroxide and the injection of formic acid are stopped, the concentration of the oxalic acid aqueous solution is also lowered. When the hydrogen peroxide concentration of the oxalic acid aqueous solution is 1 ppm or less, the valve 69 is opened to reduce the opening degree of the valve 68, and the oxalic acid aqueous solution is supplied to the cation exchange resin tower 53. As described above, the metallic cation contained in the oxalic acid aqueous solution is removed by the cation exchange resin in the cation exchange resin tower 53, and so the metallic cation concentration of the oxalic acid aqueous solution is lowered. The decomposition of oxalic acid, hydrazine and formic acid is continued in the decomposition device 55. Among oxalic acid, hydrazine and formic acid, hydrazine is decomposed first, then oxalic acid is decomposed, and formic acid remains last. In this state, the decomposition process of oxalic acid and hydrazine is completed.

When the chemical decontamination described above is completed, the purification system piping 18 is in the state shown in FIG. 4 with the oxide film containing the radionuclide removed from the inner surface of the purification system piping 18, and the inner surface of the purification system piping 18 being in contact with the aqueous solution containing formic acid described above.

The temperature of the aqueous solution is adjusted (step S3). The valves 68 and 73 are opened and the valves 69 and 74 are closed. By the drive of the circulation pumps 32 and 33, the remaining aqueous solution containing formic acid circulates in the closed loop including the circulation piping 34 and the purification system piping 18. The aqueous solution containing formic acid is heated to 90° C. by the heater 51. The temperature of the formic acid aqueous solution is desirably in a range of 60° C. to 100° C. (60° C. or more and 100° C. or less). Further, the valve 64 is opened and the valve 63 is closed. By operating these valves, the formic acid aqueous solution flowing in the circulation piping 34 is supplied to the filter 50, and fine solid content remaining in the formic acid aqueous solution is removed by the filter 50. If the fine solid content is not removed by the filter 50, a nickel metal film is formed also on the surface of the solid when a nickel metal film is formed on the inner surface of the purification system piping 18 as the nickel formic acid aqueous solution is injected into the circulation piping 34, which additionally consumes the injected nickel ions. The supply of the formic acid aqueous solution to the filter 50 is to prevent such additional consumption of nickel ions.

A surface purification agent is injected (step S4). The valve 63 is opened, the valve 64 is closed, and water flowing to the filter 50 is stopped. Since the formic acid concentration of the 90° C. aqueous solution containing the remaining formic acid is extremely low, the valve 82 is opened to drive the injection pump 80, and a formic acid aqueous solution in the chemical tank 79 containing a high concentration of formic acid as the surface purification agent is injected into the circulation piping 34 through the injection piping 81 and injected into the 90° C. aqueous solution containing the remaining formic acid flowing in the circulation piping 34. The amount of formic acid aqueous solution in the chemical tank 79 supplied to the circulation piping 34 is adjusted by controlling the rotational speed of the injection pump 80 (or opening degree of valve 82) so that the formic acid concentration of the 90° C. aqueous solution flowing in the circulation piping 34 is 30000 ppm, for example. The injection of the formic acid aqueous solution in the chemical tank 79 into the circulation piping 34 is performed before the injection of the nickel formate aqueous solution into the circulation piping 34 at step S5 described below.

A 90° C. formic acid aqueous solution (surface purification agent aqueous solution) having a formic acid concentration of 30000 ppm is supplied from the circulation piping 34 to the purification system piping 18. The surface purification agent aqueous solution 86 has a pH of 2.15, and circulates in the closed loop including the purification system piping 18 and the circulation piping 34 while contacting the inner surface of the purification system piping 18 (see FIG. 5).

When 30000 ppm of formic acid (surface purification agent) contained in the surface purification agent aqueous solution 86 is contacted with the inner surface of the purification system piping 18, iron hydroxide (Fe(OH)₃) precipitated on the inner surface is removed by the action of the formic acid (surface purification agent). There is precipitation of iron hydroxide on the inner surface of the purification system piping 18, that is, iron hydroxide is precipitated on the inner surface in the process of the step S3 performed after step S2, that is, after the chemical decontamination is completed, as the water contained in the aqueous solution circulating in the closed loop and containing the remaining formic acid is contacted with the inner surface of the purification system piping 18. In addition, since the formic acid concentration of the aqueous solution containing the remaining formic acid is extremely low, the formic acid contained in the aqueous solution cannot remove most of the iron hydroxide precipitated on the inner surface of the purification system piping 18.

Furthermore, as shown in FIG. 5, by the action of 30000 ppm of formic acid contained in the surface purification agent aqueous solution 86, dissolution of iron (II) ions (Fe²⁺) from the carbon steel purification system piping 18 on the surface purification agent aqueous solution 86 is promoted. Electrons are generated with the dissolution of iron (II) ions. For 1 hour, the surface purification agent aqueous solution 86 is contacted with the inner surface of the purification system piping 18. During this time, a large amount of iron (II) ions are dissolved from the purification system piping 18 into the surface purification agent aqueous solution 86, and a large amount of electrons are generated. The generated electrons are accumulated in the surface purification agent aqueous solution 86.

A nickel ion aqueous solution is injected (step S5). The valve 39 of the nickel ion injection device 35 is opened to drive the injection pump 37, and the nickel formate aqueous solution (formic acid concentration of 800 ppm) in the chemical tank 36 is injected into the 90° C. surface purification agent aqueous solution 86 containing 30000 ppm of formic acid flowing through the circulation piping 34 through the injection piping 38. By injecting the nickel formate aqueous solution into the surface purification agent aqueous solution 86, a film forming aqueous solution having a formic acid concentration of 30800 ppm is formed in the circulation piping 34. The pH of the film forming aqueous solution having a formic acid concentration of 30800 ppm by injection of the nickel formate aqueous solution is 2.15, which is the same as the pH of the surface purification agent aqueous solution having a formic acid concentration of 30000 ppm. The pH of the film forming aqueous solution with the nickel formate aqueous solution injected is not that greatly different from the pH of the surface purification agent aqueous solution before the nickel formate aqueous solution is injected. The nickel formate aqueous solution is injected from the chemical tank 36 into the circulation piping 34 so that the nickel ion concentration of the film forming aqueous solution is 400 ppm, for example. The 400 ppm may be realized by adjusting the nickel ion concentration of the nickel formate aqueous solution in the chemical tank 36 and the injection amount of the nickel formate aqueous solution. The nickel ion concentration of the nickel formate aqueous solution in the chemical tank 36 is adjusted by changing the mixing ratio of formic acid and nickel of nickel formate in the chemical tank 36.

A 90° C. film forming aqueous solution (film forming solution) containing nickel ions and 30800 ppm of formic acid is supplied from the circulation piping 34 to the purification system piping 18 by the drive of the circulation pump 32. The nickel ions incorporated into the inner surface of the purification system piping 18 by a substitution reaction between the iron (II) ions dissolve when this film forming aqueous solution 87 is contacted with the inner surface of the purification system piping 18, and nickel ions contained in the film forming aqueous solution 87 are instantly converted into nickel metal by the reduction action of electrons contained in the film forming aqueous solution 87, so that a nickel metal film 84 is formed on the inner surface of the purification system piping 18. In the present embodiment, hydrazine (reducing agent) is not used to form the nickel metal film 84. The film forming aqueous solution 87 discharged from the purification system piping 18 to the circulation piping 34 is pressurized by the circulation pumps 33 and 32, injected with the nickel formate aqueous solution from the nickel ion injection device 35, and injected again into the purification system piping 18. Eventually, the nickel metal film 84 is formed on the entire inner surface of the purification system piping 18 that contacts the film forming aqueous solution 87.

If the surface purification agent aqueous solution is not contacted with the inner surface of the purification system piping 18 as in Japanese Patent Laid-open No. 2018-48831, when the nickel metal film 84 covers the entire inner surface of the purification system piping 18, the dissolution of iron (II) ions from the purification system piping 18 stops, and the generation of electrons accompanying the dissolution of iron (II) ions does not occur. When electrons are no longer generated, the thickness of the nickel metal film 84 formed on the inner surface of the purification system piping 18 is not thicker than the thickness of the nickel metal film 84 that is formed on the entire inner surface of the purification system piping 18.

However, in the present embodiment, in the process of step S4, the surface purification agent aqueous solution is contacted with the inner surface of the purification system piping 18 to promote the dissolution of iron (II) ions from the purification system piping 18, and a large amount of electrons thus generated are accumulated in the surface purification agent aqueous solution. For this reason, a large amount of electrons is also accumulated in the film forming aqueous solution 87 generated by injecting the nickel formate aqueous solution into the surface purification agent aqueous solution, and after the nickel metal film 84 is formed on the entire inner surface of the purification system piping 18, the nickel ions deposited on the surface of the nickel metal film 84 are converted into nickel metal by the action of electrons contained in the film forming aqueous solution 87. Therefore, in the present embodiment, even when the entire inner surface of the purification system piping 18 is covered with the nickel metal film 84, the thickness (formation amount) of the nickel metal film 84 formed on the inner surface of the purification system piping 18 by the action of the electrons may be increased.

It is determined whether the formation of the nickel metal film is completed (step S6). When the nickel metal film 84 formed on the inner surface of the purification system piping 18 is insufficient, the processes of steps S4 to S6 are repeated. When the elapsed time after injecting the nickel formate aqueous solution into the circulation piping 34 reaches a set time (for example, 30 minutes), the injection pump 37 is stopped and the valve 39 is closed to stop the injection of the nickel formate aqueous solution into the circulation piping 34, and the formation of the nickel metal film on the inner surface of the purification system piping 18 is completed. Since the temperature of the film forming aqueous solution containing nickel ions and 30800 ppm of formic acid is 90° C., when the set time (for example, 30 minutes) elapses, the amount of nickel metal per square centimeter of the nickel metal film 84 covering the entire corresponding inner surface of the purification system piping 18 is about 2190 μg/cm². The set time (for example, 30 minutes) is obtained by measuring in advance the time until the thickness of the formed nickel metal film reaches the set thickness.

The surface purification agent is decomposed (step S7). The opening degree of the valve 68 is reduced and the valve 69 is opened, and a part of the film forming aqueous solution 87 containing nickel ions and formic acid which is a surface purification agent is guided to the cation exchange resin tower 53 through the piping 70. The nickel ions contained in a part of the film forming aqueous solution 87 are adsorbed and removed by the cation exchange resin in the cation exchange resin tower 53. By opening the valve 74 and partially closing the opening degree of the valve 73, the film forming aqueous solution 87 containing formic acid discharged from the cation exchange resin tower 53 and the film forming aqueous solution 87 containing nickel ions and formic acid that passed through the valve 68 are guided to the decomposition device 55 through the piping 75. At this time, hydrogen peroxide in the chemical tank 57 is supplied to the decomposition device 55 through the supply piping 59 and the piping 75. The formic acid contained in the film forming aqueous solution 87 is decomposed into carbon dioxide and water by the action of the activated carbon catalyst and hydrogen peroxide in the decomposition device 55.

For the surface purification agent, any of the aforementioned malonic acid and ascorbic acid may be used instead of formic acid, and both malonic acid and ascorbic acid are decomposed in the decomposition device 55 by the action of the activated carbon catalyst and hydrogen peroxide.

The film forming aqueous solution in which the surface purification agent is decomposed is purified (step S8). After the formic acid is decomposed, the valves 68 and 73 are opened, the valves 69 and 74 are closed, the supply of the film forming aqueous solution 87 to the cation exchange resin tower 53 and the decomposition device 55 is stopped, the valve 67 is opened, the opening degree of the valve 66 is partially closed, and the valve 71 is opened. At this time, the valve 69 is closed. The film forming aqueous solution 87 returned from the purification system piping 18 to the circulation piping 34, with the nickel ions removed and the formic acid decomposed, is cooled by the cooler 52 until the temperature reaches 60° C. Further, the 60° C. film forming aqueous solution 87 discharged from the cooler 52 is guided to the mixed bed resin tower 54, and nickel ions, other cations and anions remaining in the film forming aqueous solution 87 are adsorbed onto and removed by the cation exchange resin and the anion exchange resin in the mixed bed resin tower 54 (first purification process). The film forming aqueous solution cooled to 60° C. is circulated in a closed loop including the circulation piping 34 and the purification system piping 18 until the ions described above are substantially eliminated. The film forming aqueous solution that is substantially free of each ion is substantially 60° C. water. After purification, the valve 66 is opened and the valves 67 and 71 are closed.

A platinum ion aqueous solution is injected (step S9). The valve 49 of the platinum ion injection device 45 is opened and the injection pump 47 is driven. The water flowing in the circulation piping 34 is kept at 60° C. by heating by the heater 51. An aqueous solution (for example, aqueous solution of sodium hexahydroxoplatinate hydrate (Na₂[Pt(OH)₆].nH₂O)) containing platinum ions in the chemical tank 46 is injected into the water at 60° C. flowing through the circulation piping 34 through the injection piping 48. The concentration of platinum ions in this aqueous solution to be injected is 1 ppm, for example. Platinum is in an ionic state in the aqueous solution of sodium hexahydroxoplatinate hydrate. The aqueous solution containing platinum ions at 60° C. is supplied from the circulation piping 34 to the purification system piping 18 and is returned from the purification system piping 18 to the circulation piping 34 by the drive of the circulation pumps 32 and 33. The aqueous solution containing platinum ions circulates in the closed loop including the circulation piping 34 and the purification system piping 18.

In order to ensure that the platinum concentration of the aqueous solution of Na₂[Pt(OH)₆].nH₂O injected into the circulation piping 34 from the chemical tank 46 through a connection point between the circulation piping 34 and the injection piping 48 becomes the set concentration such as 1 ppm at the connection point immediately after the start of injection, the injection rate of the aqueous solution of Na₂[Pt(OH)₆].nH₂O into the circulation piping 34 is calculated in advance, and the platinum ion of 60° C. water containing ammonia flowing in the circulation piping 34 is set to the set concentration, an amount of the aqueous solution of Na₂[Pt(OH)₆].nH₂O to be filled in the chemical tank 46 that is required for the deposition of a predetermined amount of platinum on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18, is calculated, and the calculated amount of the aqueous solution of Na₂[Pt(OH)₆].nH₂O is filled in the chemical tank 46. The rotational speed of the injection pump 47 is controlled in accordance with the injection rate of the aqueous solution of Na₂[Pt(OH)₆].nH₂O into the circulation piping 34, and the aqueous solution of Na₂[Pt(OH)₆].nH₂O in the chemical tank 46 is injected into the circulation piping 34.

A reducing agent is injected (step S10). The valve 44 of the reducing agent injection device 40 is opened to drive the injection pump 42, and the aqueous solution of hydrazine which is a reducing agent in the chemical tank 41 is injected into the 60° C. aqueous solution containing platinum ions that flows through the circulation piping 34 through the injection piping 43. The hydrazine concentration of the injected hydrazine aqueous solution is 100 ppm, for example.

The hydrazine aqueous solution is injected into the circulation piping 34 after the 60° C. aqueous solution of Na₂[Pt(OH)₆].nH₂O reaches the connection point between the injection piping 43 and the circulation piping 34 that is the injection point of the hydrazine aqueous solution. In this case, the 60° C. aqueous solution containing platinum ions and hydrazine is supplied from the circulation piping 34 to the purification system piping 18. However, more preferably, it is desirable that a hydrazine aqueous solution is injected into the circulation piping 34 immediately after a predetermined amount of Na₂[Pt(OH)₆].nH₂O filled in the chemical tank 46 is completely injected into the circulation piping 34. In this case, the 60° C. aqueous solution containing platinum ions is supplied to the purification system piping 18 from the circulation piping 34, and after the injection of platinum ion aqueous solution into the circulation piping 34 is completed, 60° C. aqueous solution 88 containing platinum ions and hydrazine (see FIG. 7) is supplied from the circulation piping 34 to the purification system piping 18.

In the case of the injection of the former hydrazine aqueous solution, the reduction reaction to convert platinum ions into platinum by hydrazine first occurs in the aqueous solution 88 containing platinum ions and hydrazine flowing in the circulation piping 34, whereas in the case of injection of the latter hydrazine aqueous solution, platinum ions are previously adsorbed on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18, and the adsorbed platinum ions are reduced by hydrazine, which causes further increased amount of platinum 85 deposited on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18.

In order to ensure that the hydrazine concentration of the hydrazine aqueous solution injected from the chemical tank 41 through a connection point between the circulation piping 34 and the injection piping 43 becomes the set concentration such as 100 ppm at the connection point immediately after the start of the injection of the hydrazine aqueous solution, the injection rate of the hydrazine aqueous solution into the circulation piping 34 is calculated in advance, and hydrazine in 60° C. aqueous solution containing platinum ions flowing in the circulation piping 34 is set to the set concentration, the amount of hydrazine aqueous solution to be filled in the chemical tank 41 required for reducing the platinum ions adsorbed on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18 into platinum 85 is calculated, and the calculated amount of hydrazine aqueous solution is filled in the chemical tank 41. The rotational speed of the injection pump 42 is controlled in accordance with the injection rate of the hydrazine aqueous solution into the circulation piping 34, and the hydrazine aqueous solution in the chemical tank 41 is injected into the circulation piping 34.

When the whole amount of the aqueous solution of Na₂[Pt(OH)₆].nH₂O (aqueous solution containing platinum ions) in the chemical tank 46 is injected into the circulation piping 34, the drive of the injection pump 47 is stopped and the valve 49 is closed. Thereby, the injection of the aqueous solution containing platinum ion to the circulation piping 34 is stopped. Further, when whole entire amount of the hydrazine aqueous solution (reducing agent aqueous solution) in the chemical tank 41 is injected into the circulation piping 34, the drive of the injection pump 42 is stopped and the valve 44 is closed. Thereby, the injection of the hydrazine aqueous solution to the circulation piping 34 is stopped.

Since platinum ions adsorbed on the surface of the nickel metal film 84 are reduced into platinum 85 by the hydrazine injected, the platinum 85 is deposited on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18 (see FIG. 7). In this platinum deposition process, since the nickel metal film is already formed on the surface of the carbon steel piping, the dissolution of iron from the underlying carbon steel member is suppressed, and this facilitates the deposition of platinum.

It is determined whether or not the deposition of platinum is completed (step S11). When the elapsed time from the injection of the platinum ion aqueous solution reaches a set time, it is determined that the deposition of a predetermined amount of platinum 85 on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18 is completed. When the elapsed time does not reach the set time, the processes of steps S9 to S11 are repeated.

The aqueous solution remaining in the purification system piping 18 and the circulation piping 34 is purified (step S12). After it is determined that the deposition of the platinum 85 on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18 is completed, the valve 71 is opened and the opening degree of the valve 68 is partially closed, and a 60° C. aqueous solution containing platinum ions and hydrazine, pressurized by the circulation pump 33, is supplied to the mixed bed resin tower 54. The platinum ions, other metallic cations (for example, sodium ions), hydrazine and OH groups contained in the aqueous solution are adsorbed on the ion exchange resin in the mixed bed resin tower 54 and removed from the aqueous solution (second purification process).

The waste solution is treated (step S13). After the second purification process is completed, the circulation piping 34 and the waste solution treatment apparatus (not shown) are connected by a high pressure hose (not shown) including a pump (not shown). After completion of the second purification process, the aqueous solution that is radioactive waste solution remaining in the purification system piping 18 and the circulation piping 34 is discharged from the circulation piping 34 to a waste solution treatment apparatus (not shown) through a high pressure hose by the drive of the pump, and is treated by the waste solution treatment apparatus. After the aqueous solution in the purification system piping 18 and the circulation piping 34 is discharged, cleaning water is supplied into the purification system piping 18 and the circulation piping 34, and the circulation pumps 32 and 33 are driven to clean the inside of the piping. After completion of the cleaning, the cleaning water in the purification system piping 18 and the circulation piping 34 is discharged to the waste solution treatment apparatus.

Thus, the deposition of the nickel metal and the noble metal necessary for the conversion into the nickel ferrite film on the carbon steel member of the nuclear power plant according to the present embodiment is completed. Then, the film forming apparatus 30 connected to the purification system piping 18 is removed from the purification system piping 18, and the purification system piping 18 is reaccumulated (step S14).

According to the present embodiment, in order to contact the surface purification agent aqueous solution, for example, in order to contact the surface purification agent aqueous solution containing 30000 ppm of formic acid with the inner surface of the purification system piping 18, the dissolution amount of iron (II) ions from the purification system piping 18 on the surface purification agent aqueous solution is remarkably increased, and an increased amount of electrons are generated in connection with the dissolution of the iron (II) ions. This large amount of electrons is accumulated in the surface purification agent aqueous solution. The large amount of electrons described above is also accumulated in the film forming aqueous solution generated by injecting a nickel formate aqueous solution into the surface purification agent aqueous solution.

Until the nickel metal film 84 is formed on the entire inner surface of the purification system piping 18 in contact with the film forming aqueous solution, iron (II) ions are dissolved from the purification system piping 18 into the film forming aqueous solution and the electrons are generated.

As described above, in the present embodiment, the formation amount (thickness) of the nickel metal film 84 formed on the inner surface of the purification system piping 18 is remarkably increased by the action of a large amount of electrons (including those electrons generated when the film forming aqueous solution is contacted with the inner surface of the purification system piping 18) accumulated in the film forming aqueous solution. Then, the time required until the formation of the nickel metal film 84 on the inner surface of the purification system piping 18 is completed is significantly shortened.

In the present embodiment, a large amount of electrons that are generated in conjunction with the dissolution of the iron (II) ions generated by contacting the surface purification agent aqueous solution with the inner surface of the purification system piping 18 and accumulated in the film forming aqueous solution contributes to the formation of a nickel metal film on the inner surface of the purification system piping 18, so that it is not necessary to inject, into the film forming aqueous solution, a reducing agent that converts nickel ions into nickel metal on the inner surface of the purification system piping 18.

In the present embodiment, formation of the nickel metal film 84 covering the inner surface of the purification system piping 18 and deposition of the noble metal (for example, platinum 85) on the surface of the nickel metal film 84 may be performed using the film forming apparatus 30 as described below. This can be performed during a shut-down period of the BWR plant 1 which is before start-up of the BWR plant 1 of next operation cycle.

According to the present embodiment, since the nickel metal film 84 covering the inner surface is formed on the inner surface of the purification system piping 18 that contacts the reactor water, the dissolution of Fe²⁺ from the purification system piping 18 to the aqueous solution 88 flowing in the purification system piping 18 may be prevented, and thus the deposition of the noble metal (for example, platinum) on the inner surface of the purification system piping 18 (specifically, deposition of noble metal on the surface of the nickel metal film 84 formed on the inner surface of the purification system piping 18) is not hindered by the dissolution of Fe²⁺, and the time required for the deposition of the noble metal to the inner surface may be shortened. Further, the noble metal can be efficiently deposited on the inner surface, and the amount of the noble metal deposited on the inner surface of the purification system piping 18 is increased.

The nickel metal film 84 formed on the inner surface of the purification system piping 18 shortens the time required for the platinum to be deposited on the purification system piping 18, and, in combination with a decrease in the corrosion potential of the purification system piping 18 and the nickel metal film 84 due to the deposited platinum 85, it also contributes to the formation of a stable nickel ferrite film that does not dissolve into the reactor water by the platinum deposited on the inner surface of the purification system piping 18 as described in the third embodiment below.

The nickel metal generated from nickel ions incorporated into the purification system piping 18 by the action of electrons by the substitution reaction has very strong adhesion to the base material of the purification system piping 18. For this reason, the nickel metal film 84 is not peeled off from the purification system piping 18.

In the present embodiment, after reductive decontamination of the inner surface of the purification system piping 18, the nickel metal film 84 is formed on the inner surface of the purification system piping 18, so that the nickel metal film is not formed on the oxide film containing the radionuclide formed on the inner surface of the purification system piping 18, which reduces the radiation emitted from the purification system piping 18 and significantly reduces the surface dose rate of the purification system piping 18.

During reductive decontamination of the inner surface of the purification system piping 18 using oxalic acid aqueous solution and decomposition of oxalic acid, the iron oxalate (II) formed on the inner surface of the purification system piping 18 that is a carbon steel member is removed by the action of an oxidant (for example, hydrogen peroxide) injected into the oxalic acid aqueous solution. By removing the iron oxalate (II), the adhesion between the purification system piping 18 and the nickel metal film 84 is improved, and the nickel metal film 84 may be prevented from peeling off from the inner surface of the purification system piping 18.

Second Embodiment

A method for depositing a noble metal on a carbon steel member of a nuclear power plant according to a second embodiment applied to a purification system piping of a boiling water type nuclear power plant, which is another preferred embodiment of the present invention, will be described with reference to FIGS. 9 and 10. The method for suppressing the deposition of radionuclides on a carbon steel member of a nuclear power plant according to the present embodiment is applied to a carbon steel purification system piping (carbon steel member) of the BWR plant.

In the present embodiment, as shown in FIG. 9, the processes of steps S1 to S3 and S6 to S14 in the method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the first embodiment, and new process of step S4A are performed. The process of step S4A is a process that is performed instead of each of the processes of steps S4 and S5, for injecting an aqueous solution containing a surface purification agent and nickel ions into the circulation piping 34.

A film forming apparatus 30A shown in FIG. 10 is used in the method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the present embodiment. Unlike the film forming apparatus 30 used in the first embodiment, the film forming apparatus 30A includes a device 90 for injecting surface purification agent and nickel ion. The configuration of the film forming apparatus 30A except the device 90 for injecting surface purification agent and nickel ion is the same as the configuration of the film forming apparatus 30 except the nickel ion injection device 35 and the formic acid injection device 78.

The device 90 for injecting surface purification agent and nickel ion includes the nickel ion injection device 35, the formic acid injection device 78, and an injection piping 91. Each of the injection piping 38 of the nickel ion injection device 35 and the injection piping 81 of the formic acid injection device 78 is connected to one end of the injection piping 91. The other end of the injection piping 91 is connected to the circulation piping 34 between a connection point between the injection piping 48 and the circulation piping 34 and the valve 76. In the present embodiment, the injection pipings 91, 48 and 43 are arranged in this order from the valve 76 toward the on-off valve 77 and connected to the circulation piping 34.

The process of step S4A will be specifically described below.

After performing the processes of steps S1 to S3, an aqueous solution containing a surface purification agent and nickel ions is injected (step S4A). The valves 39 and 82 are opened to drive the injection pumps 37 and 80. The nickel formate aqueous solution in the chemical tank 36 is supplied to the injection piping 91 through the injection piping 38, and the surface purification agent aqueous solution containing formic acid in the chemical tank 79 is further supplied to the injection piping 91 through the injection piping 81. The nickel formate aqueous solution and the surface purification agent aqueous solution are mixed in the injection piping 91 to produce a film forming aqueous solution. This film forming aqueous solution is guided from the injection piping 91 to the circulation piping 34 and injected into the 90° C. aqueous solution containing the remaining formic acid flowing in the circulation piping 34. The 90° C. film forming aqueous solution containing nickel ions and formic acid in the circulation piping 34 is supplied to the purification system piping 18. The flow rate of the nickel formate aqueous solution supplied from the chemical tank 36 to the injection piping 91 is adjusted so that the nickel ion concentration of the film forming aqueous solution in the circulation piping 34 is 400 ppm, for example, and the flow rate of the surface purification agent aqueous solution supplied from the chemical tank 79 to the injection piping 91 is adjusted so that the formic acid concentration of the film forming aqueous solution in the circulation piping 34 is 30000 ppm, for example.

The 90° C. film forming aqueous solution containing 400 ppm of nickel ions and 30000 ppm of formic acid is contacted with the inner surface of the purification system piping 18. Iron (II) ions are dissolved from the purification system piping 18 into the film forming aqueous solution by the action of the formic acid at such concentration, and electrons are generated. The nickel ions in the film forming aqueous solution are incorporated into the inner surface of the purification system piping 18 by the substitution reaction with iron (II) ions. The incorporated nickel ions are reduced by the above electrons and converted into nickel metal. When the 90° C. film forming aqueous solution containing 400 ppm of nickel ions and 30000 ppm of formic acid is contacted with the inner surface of the purification system piping 18, nickel metallization of nickel ions incorporated into the inner surface of the purification system piping 18 is promoted, and eventually, the entire inner surface of the purification system piping 18 that contacts the film forming aqueous solution is covered with a nickel metal film. As a result, the dissolution of iron (II) ions from the purification system piping 18 stops, and the generation of electrons accompanying the dissolution also stops. Meanwhile, as long as there are electrons present in the film forming aqueous solution, nickel ions deposited on the formed nickel metal film are converted into nickel metal, but eventually the conversion of the nickel ions into nickel metal also stops.

When the determination at step S6 is “YES”, the formation of the nickel metal film is completed, and then the processes of steps S7 to S14 are sequentially performed.

In the present embodiment, each effect produced in the first embodiment may be obtained. In the present embodiment, the thickness of the nickel metal film formed on the inner surface of the purification system piping 18 is thinner than the thickness of the nickel metal film formed in the first embodiment, but the time required for forming this nickel metal film is shorter than the time required for forming the nickel metal film in Japanese Patent Laid-open No. 2018-48831.

Even when the film forming apparatus 30 shown in FIG. 3 is used without using the device 90 for injecting surface purification agent and nickel ion, it is possible to produce a film forming aqueous solution containing the nickel formate aqueous solution and the surface purification agent aqueous solution in the second embodiment, that is, to produce a 90° C. film forming aqueous solution containing 400 ppm of nickel ions and 30000 ppm of formic acid. Specifically, the 90° C. film forming aqueous solution containing 400 ppm of nickel ions and 30000 ppm of formic acid is produced in the circulation piping 34 by, at the film forming apparatus 30 connected to the purification system piping 18, injecting formic acid aqueous solution into 90° C. aqueous solution containing the remaining formic acid flowing in the circulation piping 34 from the upstream formic acid injection device 78, producing 90° C. surface purification agent aqueous solution containing 30000 ppm of formic acid in the circulation piping 34, and when this aqueous solution reaches the connection point between the injection piping 38 and the circulation piping 34 of the nickel ion injection device 35, injecting the nickel formate aqueous solution from the nickel ion injection device 35 into the circulation piping 34, and injecting this nickel formate aqueous solution into 90° C. surface purification agent aqueous solution containing 30000 ppm of formic acid flowing in the circulation piping 34. With the injecting into the 90° C. surface purification agent aqueous solution containing 30000 ppm of formic acid, the 90° C. film forming aqueous solution containing 400 ppm nickel ions and 30000 ppm of formic acid is supplied to the purification system piping 18 in the same manner as in second embodiment.

Third Embodiment

A method for suppressing deposition of radionuclides on a carbon steel member of a nuclear power plant according to a third embodiment applied to a purification system piping of a boiling water type nuclear power plant, which is another preferred embodiment of the present invention, will be described with reference to FIG. 11. The method for suppressing the deposition of radionuclides on a carbon steel member of a nuclear power plant according to the present embodiment is applied to a carbon steel purification system piping (carbon steel member) of the BWR plant.

In the present embodiment, as shown in FIG. 11, the processes of steps S1 to S14 in the method for depositing a noble metal on a carbon steel member of a nuclear power plant according to the first embodiment and new processes of steps S15 to S16 in addition to these processes are performed.

After the processes of steps S1 to S14 are performed, the processes of steps S15 and S16 are performed. The processes of steps S15 and S16 will be specifically described below.

The nuclear power plant is started (step S15). After completion of fuel change and maintenance of the BWR plant 1, in order to start the operation of the next operation cycle, the BWR plant 1 having the purification system piping 18 formed with, on the inner surface thereof, the nickel metal film 84 with the platinum 85 deposited thereon, is started.

Reactor water at 130° C. or more is contacted with the nickel metal film on which platinum is deposited (step S16). When the BWR plant 1 is started, the reactor water present in the downcomer in the RPV 3 is supplied to the reactor core 4 through the recirculation system piping 6 and the jet pump 5 as described above. The reactor water discharged from the reactor core is returned to the downcomer. The reactor water in the downcomer flows into the purification system piping 18 through the recirculation system piping 6 and eventually flows into the feedwater piping 11 and is returned to the RPV 3.

A control rod (not shown) is pulled out from the reactor core 4 to change the reactor core 4 from a subcritical state to a critical state, and the reactor water in the reactor core 4 is heated by heat generated by fission of nuclear fuel material in the fuel rod. The steam is not generated in the reactor core 4. Further, the control rod is pulled out from the reactor core 4, and the pressure in the RPV 3 is increased to the rated pressure in the process of elevating the temperature and pressure of the nuclear reactor 2, and the reactor water is heated by the heat generated by the fission, so that the temperature of the reactor water in the RPV 3 rises to the rated temperature (280° C.). After the pressure in the RPV 3 reaches the rated pressure and the reactor water temperature rises to the rated temperature, the control rod is further pulled out from the reactor core 4 and the flow rate of the reactor water supplied to the reactor core 4 is increased, so that the reactor power is increased to the rated power (100% power). The rated operation of the BWR plant 1 while maintaining the rated output is continued until the operation cycle is completed. When the reactor power rises to 10% power, for example, steam generated in the reactor core 4 is supplied to the turbine 9 through the major steam piping 8, and power generation is started.

The reactor water contains oxygen and hydrogen peroxide. The oxygen and hydrogen peroxide are generated by radiation decomposition of reactor water in RPV3. The oxygen-containing reactor water 89 in the RPV 3 is guided from the recirculation system piping 6 into the purification system piping 18 in a state in which the purification system pump 19 is driven, and is contacted with the nickel metal film 84 which is formed on the inner surface of the purification system piping 18 and on which the platinum 85 is deposited (see FIG. 12). By the heating of the reactor water 89 by the heat generated by the fission described above, the temperature of the reactor water 89 in contact with the nickel metal film 84 rises to eventually reach 130° C. or more, and finally rises to 280° C. at the rated output.

The temperature of the reactor water 89 varies greatly before and after the regenerative heat exchanger 20 and the non-regenerative heat exchanger 21. When the temperature of the reactor water in the RPV 3 is 280° C., the reactor water 89 at about 280° C. flows through the purification system piping 18 upstream of the regenerative heat exchanger 20. As a result of heat exchange in the regenerative heat exchanger 20, the temperature of the reactor water 89 flowing out from the regenerative heat exchanger 20 toward the valve 25 is lowered to a range of about 200° C. to 150° C. Furthermore, in the non-regenerative heat exchanger 21, the reactor water 89 is lowered to a temperature within a range from 50° C. to about room temperature, and is supplied to the reactor water purification device 22 containing the ion exchange resin within this temperature range. Since the reactor water 89 flowing out of the reactor water purification device 22 is used as the feedwater, it is heated in the range of about 150° C. to 200° C. by the regenerative heat exchanger 20 and then joins the feedwater flowing through the feedwater piping 11.

During the period of time when the BWR plant 1 is started and the pressure in the RPV 3 rises to the rated pressure (the temperature of the reactor water at this time is 280° C.), the reactor water 89 flowing through a portion of the purification system piping 18 between the valve 23 and the regenerative heat exchanger 20, the reactor water 89 flowing through a portion of the purification system piping 18 between the regenerative heat exchanger 20 and the valve 25, and the reactor water 89 flowing through a portion of the purification system piping 18 closer to the feedwater piping 11 than the regenerative heat exchanger 20, have a temperature of 130° C. or more, although there may be time lag. In the process of elevating the temperature and pressure of the reactor 2, as the pressure in the RPV 3 increases, the temperature of the reactor water 89 in the RPV 3 rises above 130° C. to a higher temperature.

Therefore, the surface of the nickel metal film 84 which is formed on the inner surface of the purification system piping 18 between the valve 23 and the valve 25 and on which the platinum 85 is deposited is contacted with the oxygen-containing reactor water 89 at a temperature range of 130° C. or more to 280° C. or less, so that the purification system piping 18 and the nickel metal film 84 thereof are heated to the same temperature as the reactor water 89. Oxygen contained in the reactor water 89 is migrated into the nickel metal film 84 formed on the inner surface of the purification system piping 18 between the valve 23 and the valve 25, and Fe contained in the purification system piping 18 which is a carbon steel member is converted into Fe²⁺ and is migrated into the nickel metal film 84 (see FIG. 13). In a high temperature environment at a temperature range of 130° C. or more and 280° C. or less, oxygen contained in the reactor water 89 and Fe²⁺ from the purification system piping 18 are likely to be migrated into the nickel metal film. When the oxygen concentration in the reactor water 89 is low, water molecules in the reactor water 89 are decomposed by iron corrosion to produce oxygen, and this oxygen serves the same function as the oxygen contained in the reactor water 89 described above. By the lowered corrosion potential of the purification system piping 18 and the nickel metal film 84 by the action of platinum 85 deposited on the nickel metal film 84, and by the formation of a high temperature environment at a temperature range of 130° C. or more to 280° C. or less, the nickel metal film 84 reacts with oxygen and Fe²⁺ migrated into the nickel metal film 84, and a stable nickel ferrite (NiFe₂O₄) in which x is 0 in Ni_(1−x)Fe_(2+x)O₄ is produced.

In the process of elevating the temperature and pressure, from the stage before the temperature of the reactor water 89 reaches 130° C., hydrogen is injected into the feedwater flowing through the feedwater piping 11 by a hydrogen injection device (not shown) connected to the feedwater piping 11 between the condensate pump 12 and the condensate purification device 13. This hydrogen injection is performed during the process of elevating the temperature and pressure and the process of increasing the reactor power, and during the rated operation of the BWR plant 1. Since the feedwater containing hydrogen is supplied to the reactor pressure vessel 3, the hydrogen is eventually injected into the reactor water. The lowered corrosion potential of each of the purification system piping 18 and the nickel metal film 84 by the action of the platinum 85 described above occurs when oxygen contained in the reactor water 89 and the injected hydrogen react with each other by the action of the platinum 85 and thus water is formed.

When stable nickel ferrite is produced, whether nickel or iron is incorporated easily into the ferrite structure is influenced by platinum (noble metal) 85, such that when platinum 85 is present, nickel is more likely to be incorporated into than iron, so that stable nickel ferrite in which x is 0 in Ni_(1−x)Fe_(2+x)O₄ is produced. Then, the nickel metal film 84 formed on the inner surface of the purification system piping 18 is converted into a stable nickel ferrite (NiFe₂O₄) film 86A, and the inner surface of the purification system piping 18 between the valve 23 and the valve 25 is covered with the stable nickel ferrite film 86A having platinum 85 deposited on the surface (see FIG. 14).

In the present embodiment, each effect produced in the first embodiment may be obtained. Furthermore, the present embodiment may also obtain the following effects.

In a high temperature environment at a temperature range of 130° C. or more and 280° C. or less, the nickel ferrite (NiFe₂O₄) produced as described above in which x is 0 in Ni_(1−x)Fe_(2+x)O₄ has significant growth of crystals from the nickel metal film 84 covering the inner surface of the purification system piping 18, and despite the noble metal deposited, does not dissolve into water like Ni_(0.7)Fe_(2.3)O₄ film, but remains stable, and may suppress the deposition of radionuclide on the carbon steel which is the base material, that is, on the purification system piping 18.

In the present embodiment, as described in the first embodiment, since the formation amount (thickness) of the nickel metal film formed on the inner surface of the purification system piping 18 increases, the thickness of the stable nickel ferrite film on the inner surface of the purification system piping 18 converted from the nickel metal film also increases. For this reason, according to the present embodiment, the deposition of the radionuclide on the purification system piping 18 may be further suppressed.

In the present embodiment, since the nickel metal film 84 is formed on the inner surface after the reductive decontamination with respect to the inner surface of the purification system piping 18 is completed, the nickel metal film 84 may be prevented from being peeled off from the inner surface of the purification system piping 18.

According to the present embodiment, the nickel ferrite film 86A produced from the nickel metal film 84 as described above, in which x is 0 in Ni_(1−x)Fe_(2+x)O₄, by the action of platinum 85 deposited on the nickel metal film 84 and under a high temperature environment of 130° C. or more and 280° C. or less is a stable nickel ferrite film that does not dissolve into the reactor water by the action of the deposited platinum 85 even during operation of the BWR plant 1. The stable nickel ferrite film 86A produced as described above, which does not dissolve into the reactor water by the action of the deposited platinum 85, may suppress the corrosion of the purification system piping 18 over a longer period of time than the Ni_(0.7)Fe_(2.3)O₄ film produced at a low temperature range of 60° C. to 100° C. Specifically, the stable nickel ferrite film 86A formed on the inner surface of the purification system piping 18 does not dissolve by the action of the deposited platinum 85, and may cover the inner surface of the purification system piping 18 over a plurality of operation cycles, for example, for five operation cycles (for example, five years). Thus, since the stable nickel ferrite film 86A can cover the inner surface of the purification system piping 18 over a long period of time, the purification system piping 18 is prevented from deposition of the radionuclide over a long period of time.

In the present embodiment, the nickel metal film 84 formed on the inner surface of the purification system piping 18 not only shortens the time required for the platinum to be deposited on the purification system piping 18, but also contributes to the formation of a stable nickel ferrite film 86A having the action of the deposited platinum, without dissolving into the reactor water by the action of the platinum 85 deposited on the inner surface of the purification system piping 18. The nickel ferrite film 86A prevents the reactor water flowing through the purification system piping 18 from contacting with the base material of the purification system piping 18 after the start of the BWR plant of a next operation cycle. For this reason, corrosion of the purification system piping 18 due to the reactor water is suppressed, and further, the radionuclide contained in the reactor water is not incorporated into the base material of the purification system piping 18.

The processes of steps S15 and S16 according to the present embodiment may be performed subsequent to the process of step S14 after the process of step S14 is completed in the second embodiment.

Further, each of first to third embodiments can be applied to a carbon steel piping connected to a reactor pressure vessel in a pressurized water nuclear power plant. The temperature of the reactor water in the reactor pressure vessel of the pressurized water nuclear power plant is higher than the temperature of the reactor water in the reactor pressure vessel 3 in the boiling water type nuclear power plant. 

What is claimed is:
 1. A method for depositing a noble metal on a carbon steel member of a nuclear power plant, the method comprising: contacting a film forming solution containing nickel ions and a surface purification agent and having a pH within a range of 1.8 or more to 2.5 or less with a first surface of a carbon steel member of a nuclear power plant that contacts reactor water; and forming a nickel metal film on the first surface; and depositing a noble metal on a second surface of the formed nickel metal film, wherein the forming the nickel metal film and the depositing the noble metal are performed after shut-down of the nuclear power plant and before start-up of the nuclear power plant.
 2. A method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 1, wherein the formation of the nickel metal film including the first step of contacting the film forming solution containing the nickel ions and the surface purification agent with the first surface and the second step of forming the a nickel metal film on the first surface is performed by steps of: contacting a surface purification agent solution containing a surface purification agent and having a pH within a range of 1.8 or more to 2.5 or less with a first surface of a carbon steel member of a nuclear power plant that contacts reactor water; after a set time is elapsed since the surface purification agent solution is contacted with the first surface of the carbon steel member, injecting nickel ions into the surface purification agent solution to produce a film forming solution to produce the film forming solution; and contacting the produced film forming solution with the first surface of the carbon steel member to form a nickel metal film on the first surface.
 3. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 1, wherein any one of formic acid, malonic acid, and ascorbic acid is used as the surface purification agent.
 4. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 1, wherein the forming the nickel metal film is performed by reducing the nickel ions incorporated into the first surface of the carbon steel member by using electrons generated when iron ions are dissolved from the carbon steel member by the surface purification agent.
 5. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 4, wherein the depositing the noble metal on the second surface of the nickel metal film is performed by contacting an aqueous solution containing noble metal ions and a reducing agent with the second surface of the nickel metal film.
 6. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 1, wherein the contacting the film forming solution with the first surface of the carbon steel member is performed after removing an oxide film formed on the carbon steel member.
 7. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 2, wherein the contacting the surface purification agent solution with the first surface of the carbon steel member is performed after removing an oxide film formed on the carbon steel member.
 8. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 6, wherein the removing the oxide film is performed by contacting an oxalic acid aqueous solution with the first surface of the carbon steel member, wherein an oxidant is injected into the oxalic acid aqueous solution.
 9. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 1, comprising decomposing the surface purification agent contained in the film forming solution after the nickel metal film is formed and before the noble metal is deposited on the second surface of the nickel metal film.
 10. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 1, wherein the forming the nickel metal film is performed by supplying, through a second piping, the film forming solution to a first piping which is connected to a reactor pressure vessel and which is the carbon steel member, and by contacting the film forming solution with an inner surface of the first piping, which is the first surface of the carbon steel member.
 11. The method for depositing a noble metal on a carbon steel member of a nuclear power plant according to claim 2, wherein the contacting the surface purification agent solution with the first surface of the carbon steel member is performed by supplying, through a second piping, the surface purification agent solution to a first piping which is connected to a reactor pressure vessel and which is the carbon steel member, and by contacting the surface purification agent solution with an inner surface of the first piping, which is the first surface of the carbon steel member, and the forming the nickel metal film is performed by supplying the film forming solution produced in the second piping by injecting the nickel ions into the surface purification agent solution to the first piping and by contacting the produced film forming solution with the inner surface of the first piping, which is the first surface of the carbon steel member.
 12. A method for suppressing deposition of radionuclides on a carbon steel member of a nuclear power plant, the method comprising: contacting a film forming solution containing nickel ions and a surface purification agent and having a pH within a range of 1.8 or more to 2.5 or less with a first surface of a carbon steel member of a nuclear power plant that contacts reactor water; and forming a nickel metal film on the first surface; and depositing a noble metal on a second surface of the formed nickel metal film, wherein the forming the nickel metal film and the depositing the noble metal are performed after shut-down of the nuclear power plant and before start-up of the nuclear power plant, and contacting oxygen-containing water having a temperature range of 130° C. or more and 330° C. or less with a second surface of the nickel metal film on which the noble metals are deposited.
 13. The method for suppressing deposition of radionuclides on a carbon steel member of a nuclear power plant according to claim 12, the method comprising: starting the nuclear power plant after the noble metals are deposited on the second surface; and using reactor water as the oxygen-containing water having the temperature range of 130° C. or more and 330° C. or less. 